While they are explaining this to me, as we sit in the light-filled sitting room of the lower apartment, Susie’s twin sister Jen, with whom she runs a film location catering company, is upstairs making the last tweaks to the upper flat to make it camera perfect. So you have twin flats, twin sisters and a lovely warm family feeling to it all.

There’s also a bit of a magical air to the characterful Edwardian building the flats are in, which has a stunning semi-circular ‘captain’s window’ in the top flat, looking across a small park and out to sea. The stairway from the lower flat up to Susie’s original bolt hole, adds to that feeling.

There’s something rather Famous Five about walking out of one front door, scuttling up the stairs – past the wonderful ‘steam trawler’ sign Susie bought in an antique shop in Norman Road – and in through the front door of another flat. It would be very easy to imagine doing it in pyjamas… with a lot of giggling.

And there also seemed to have been a sprinkling of fairy dust about the way Susie found the original flat ten years ago.

“When my daughter was four,” says South London-based Susie, “I began to think it would be lovely to have somewhere by the sea to go to at weekends. Me and my three sisters spent our childhood holidays with our grandparents in Lyme Regis, but that was too far to go from London for a weekend.

“One day I just had this inspiration that I should look in Hastings. That very afternoon I opened the Evening Standard and in the homes section there was a picture of a flat in Hastings with a lovely window, for sale.”

The next morning she was standing in it and thinking it was even better than it had looked in the paper.

“But I thought, ‘I can’t buy the first place I look at…’ so I dragged my four-year-old round five more flats – and then came back and bought the first one.”

Adding to the luck of that chance sighting was that Susie bought the flat from Andy and Christine Cowle, of the well-known Bexhill antiques emporium Eras of Style.

“I was so lucky,” says Susie, “because it was already impeccably decorated by Andy and Christine and I was able to buy some of the furniture from them, so it was all there waiting for us.”

While that flat was very much bought to be a seaside escape for Susie and her daughter, that little girl is now a teenager and more interested in spending her weekends at home, where her school friends are, than eating ice creams on the beach.

So it was another stroke of luck when, two and a half years ago, just as Susie was starting to rent the flat out on holidaylettings.co.uk and Airbnb – managed by her friend Sarah Thompson – the one below came on the market. Susie could immediately see the potential for letting them both out – either separately, or together for one larger party, joined by those quirky stairs.

Big sister Caroline, a north London-based garden designer, immediately saw the potential – and was excited about the prospect of refurbishing it as a joint project, which was a good thing, considering the state it was in…

“It was filthy… in a terrible state,” says Susie. “We gutted it and put in a new kitchen and bathroom and stripped the doors and floors to freshen it.”

“We really wanted to please ourselves,” adds Caroline. “To make something completely from scratch with a contemporary/vintage feel.”

Looking around the airy room, painted a beautiful pale sea blue and drenched in that special seaside light, it’s easy to imagine what a haven it would be for a stressed city dweller, sorely needing a break – which was exactly their aim. “We wanted to keep it simple so every piece in here had to earn its place,” says Caroline. “We didn’t want it to be cluttered, because that’s not relaxing.”

Even the sofa, from SofasWorld, was chosen with relaxing holiday makers in mind. “We wanted something really big,” says Caroline, “for people to lie on and read a book.”

While that sofa is from a big online company, the vast majority of the furnishings in both flats were sourced close to home – whether that is home defined as within walking distance of the St Leonards flats, favourite emporia near their London houses, or legacies from their parents’ and grandparents’ family homes.

A quick glance round the sitting room tells the story. The glass side table is from The Kula, the fish print from the yard in Courthouse Street and the light fitting from Nelson Antiques, all in Hastings Old Town. The glasses on the table are from Eras of Style, the wall sconces from an antique shop on Norman Road and the seagull artwork from the gallery in Marine Court. Local heroes all.

The side tables are from a shop near Caroline’s home in Highgate and the standard lamp from Angelo’s Lighting – an iconic north London source. And if they couldn’t find what they wanted, near home or online, they had it made.

“The dining table was too high,” says Susie, “or perhaps the chairs we had were too low… Either way, it wasn’t working so we had Patrick Seaman from Werewolves of London make us some chairs specially, with a bench to go opposite.”

Heading on to look at the nearest bedroom we pass by the kitchen, which is lovely and light from a large aperture they had put through the wall to the sitting room. On the other side, a utility area leads out onto a delightful balustraded balcony, one end panelled with tongue and groove painted a very marine shade of blue. It’s the perfect place to sit and enjoy the view and the sea air. “It gets afternoon sun,” says Caroline, “and the house next door provides perfect shelter from the prevailing south westerly wind.”

While it wouldn’t be right to call the bedroom behind the terrace the ‘master bedroom’ because all three of them are special in both flats, it does have a very specific and thrilling feature: a raised ‘princess and the pea’ bed.

They commissioned this from Mike and Kerry of Woodworks 1066 – a business so local, it’s a four-minute drive from the flats – to be high enough for guests to able to sit and look out of the window at the sea from it. There is storage underneath and steps up, which would make you feel very special getting in to it. Woodworks 1066 also made the mirror mounted on the chimney breast beside the bed.

The bedside tables were customised by their builder John Gair, who did all the work on both apartments, stripping them and adding shells to the top, to reflect the octagonal table with shells under glass, by the window, which Susie found in a rummage-about second hand shop on London Road, St Leonards.

A Lloyd Loom chair and a chest of drawers are the only other pieces of furniture in the room – a deliberate choice inspired by their intention to keep the space as uncluttered and restful as possible

“People are staying for a week maximum,” says Caroline, “so they don’t need a lot of storage – just a chest of drawers and the row of hooks to hang things on. We store the spare bed linen in the drawers under the bed. It keeps the light and space in the room, which adds to the sense of relaxation.”

The first of the back bedrooms, which looks onto the garden and the dramatic rock face of the hill behind the terrace of houses, has an equally calm atmosphere, although it is furnished more richly with lovely old pieces, in charming period keeping with the lilac-painted walls. Particularly striking is the antique kilim which Susie found at a well-known antiques centre near her home in Crystal Palace.

The leaf print next to the bookshelves illustrates another theme of the décor of both flats; it’s been lurking in Susie’s garden shed for years. In a similar vein, the maps on the adjacent wall belong to Caroline’s husband and were liberated from their attic.

Completing their range of sources in this one room, the eiderdown was their grandmother’s and the art deco mirror over the fireplace was bought from Eras of Style.

Moving on to the twin bedroom, with its lovely fern theme, inspired by the vegetation in the garden it overlooks, the same mix of sources applies. The Victorian mirror was a find by Caroline from an antiques centre equivalent to Susie’s spot in Crystal Place, in her north London ’hood, while the magnificent brass curtain rail and hooks are from Susie’s shed stash.

Other furnishings in this room reflect their use of local businesses. The fern wallpaper is from Vintage Bird in Hastings Old Town and while the curtains are from Next, they had them lengthened to reach the floor by getting the needleworkers at Little Sew and Sew in St Leonards, just five minutes walk along the road, to add some green fabric at the bottom.

This ultra local sourcing continues, in the bathroom just across the hallway, with vinyl from what is literally the very nearest flooring shop. It’s an excellent way to shop, if you don’t want to spend precious free time schlepping about and even though they bought all the bathroom fittings from a bathroom supply shop in London – it’s one Susie’s used for every bathroom she’s renovated. Local and/or loyal is the mantra here.

“We wanted an Edwardian look for the bathroom,” explains Caroline, “in keeping with the period of the building, not a modern bathroom. But we did want a bigger shower, because that’s nice when you’ve come from the beach, so we put in a smaller bath to accommodate it. The key thing is people still have the choice.”

The mood of the upstairs flat is pleasingly the same – but different. The tongue and groove panelling which runs hip height along both hallways up here is painted a sunny bright yellow, downstairs it’s a sophisticated mulberry.

The set up of a lovely wooden dining table with chairs on one side and a bench seat on the other, with a splendid painting hanging above it, is like a mirror image of downstairs, except these pieces are antiques, which Susie bought in situ from Andy and Christine of Eras of Style.

That amazing captain’s window is the stand out feature of the room, with two lovely squashy sofas, from sofa.com, for double lounging with a book potential.

In the characteristic mash up of sources, the mid-century sideboard is another find from Susie’s Crystal Palace emporium, the armchair upholstered with a kilim was found in Hastings Old Town and the wheatsheaf wall light from Sideshow Interiors in Norman Road.

The wonderful carved wooden throne at the head of the dining table belonged to their father and the splendid Turkish rug was a gift from a friend, who found it at the oriental carpet specialist in West Peckham.

Just as in the downstairs flat, the three bedrooms on this floor each have their own, very appealing personality (and there’d be none of those classic holiday fights over ‘who’s having the naff room?’).

The front bedroom, painted the same light sea blue as the sitting room downstairs, is very light and feminine, with a white fireplace and over mantle mirror, a lovely old brass bed and a temptingly squashy armchair. As in its equivalent directly below, there’s a shell table, this one filled with beach treasures picked up by their mother ‘all over the world’.

The bedroom behind it has a complementary masculine feel, with the air of a gentleman’s club in the leather chair, velvet curtains, horse painting and the ‘New Antique Books’ wallpaper by St Leonard-based wallpaper designer Deborah Bowness. Most of the furniture and objects in this room are from their father’s house.

The final bedroom is very much a child’s sanctuary, featuring a dolls’ house made for the daughter of their sister Jen. Susie found the lovely wooden shutters in a skip – and deliberately didn’t have them renovated.

Let out together the flats sleep twelve and it’s impossible not to imagine excited children running up and down between the two – perhaps taking turns to hop up the steps into the princess and the pea bed – in a spirit of close family fun very much in keeping with the owners.