Martin Brudnizki's 17th-century Sussex flat decked out in colourful Christmas finery

Binderton House was once the country residence of the former Prime Minister Anthony Eden, who owned it between 1941 and 1952 (during which time he held various posts in the Foreign Office). The core of the house is a handsome two-storey building, dating back to 1677, with a pedimented stone porch and wings of different lengths added to both ends in the 18th century. It was split into seven flats in the 1980s and feels more like a little hamlet than a conversion. The flat Martin and Jonathan viewed was on the ground floor in the oldest and grandest part of the house: a two-bedroom home, which includes the original drawing room of the house.

And oh, what a room. By the time they were back in London, Martin was already imagining what he could do with it. He had conjured the idea of ‘creating our own Arcadian fantasy’. This is a word that crops up a few times during my visit. ‘This place is all about fantasy,’ Martin says. ‘I wanted it to look as though, in the 1930s, the fictitious earl that lived here married someone – probably an American with lots of money – and she redecorated the house.’ That gives some insight into the mind of this designer.

Although Binderton House is Grade II* listed, they were able to rearrange some of the rooms and move the odd doorway to create a more rationalised floor plan. As a result, it is so much more than the sum of its parts, feeling grander than any two-bedroom flat has a right to. But good proportions and a clever layout can do that.

The main event is the enormous drawing room. At one end, a dozen people can easily sit, perch, lean and lounge around the fireplace; towards the three-window bay, which overlooks the back garden, is a dining table large enough to seat 10. ‘I wanted a really social set-up,’ Martin explains. ‘That was important to me.’

He also knew he wanted it to be yellow – ‘like sunshine even on a dark winter’s day’. As daylight fades, it takes on a golden hue and the room simply glows, thanks in no small part to the dimmed lights and candles – there are no fewer than 20 of the former and 20 of the latter. The light catches the now-gilded plasterwork cornice and original pine fire surround, stripped of layers of paint, part-gilded and festooned at this time of year with evergreen garlands.

‘Friends say, “You have so much stuff,” and I think, “There isn’t enough!” ’ says Martin with a chuckle. As for Jonathan, he somehow has the air of being both long-suffering and completely supportive. ‘I kept telling Martin off for adding more flourishes,’ he says. ‘Then he’d do it and I’d say, “Actually, that’s great. Don’t listen to me.” ’

To be clear, Jonathan is no innocent bystander. He played a key role in amassing the pictures and devising the exuberant curation: a series of dog portraits here, a collection of Italian flags there. There is a ‘Soane moment’ featuring plaster casts in the hallway. Pictures hang from thick ribbons and rosettes, in part inspired by the late American ‘Prince of Chintz’ – the decorator Mario Buatta.

If you ever find yourself wondering what to do with a kitchen that has 3.7-metre ceilings, the answer, it seems, is to create four finial-topped towers and place them in the corners of the room. These tall cabinets disguise the fridge-freezer, the washer and dryer, and lots of storage, and add a sense of drama to a room that also features a veined white marble splashback creeping high up an orange wall. This is set off by a collection of pewter and porcelain plates, among them some once owned by Nancy Lancaster, and balanced by an antique tapestry on the adjacent wall.

The bedrooms are a full-throated celebration of fabric and detailing. In the main bedroom, Jean Monro’s foxglove-patterned ‘Lustmore’ linen takes centre stage: it covers the walls and is used for the curtains, pelmet and bedcover. The blue-green from the print reappears in the Samuel & Sons trimmings, while the pink is repeated on the ceiling, in ‘Rose’ by Edward Bulmer Natural Paint. Side note: Edward Bulmer’s website says this colour will ‘give your bedroom some pizzazz’. And that is no lie. The tented ceiling and the walls of the spare room are in a combination of two fabrics designed by And Objects (the product design business that Martin set up with Nicholas Jeanes in 2015) for Christopher Farr Cloth, and here there is an even more lavish use of Samuel & Sons trims.

The pair now spend more time in the country than London. ‘For me, this is home,’ says Martin, who will pop into town a couple of days a week for work. ‘We’ve made some great friends and settled into life down here.’ One of their favourite walks is through wood-land past a Saxon hill fort, then looping around West Dean, taking in views of the coast. Over Christmas, they will visit local markets, attend evensong at Chichester Cathedral and watch the Boxing Day hunt on the nearby Petworth estate. They will cook and entertain in that magnificent room in their own slice of an English country house. ‘I think we know that this is a moment in our lives and we can enjoy it,’ says Jonathan. Martin finishes the thought, ‘We’d still love to buy a house in the country one day, but it will never be one quite like this. So, for now, we are living the fantasy’

MBDS: mbds.com