Spencer House: One of London's most remarkable, hidden jewels

Winter weekends are custom-made for city breaks and indoor sightseeing. London's museums know this well, with November through February being a favourite time for blockbuster exhibitions. A day wandering the V&A or the British Museum will always banish the gloom that short, dark days wrap around me. It's worth remembering, however, that London has many gems beyond the usual blockbusters, and January and February are ideal times to explore. With tourists at their seasonal low, you might even find that you have a treasure almost to yourself.

Such was the case recently at Spencer House, arguably one of the greatest Georgian interiors in the country yet little known beyond serious fans of the age. Its location couldn't be more central ... overlooking Green Park, almost within shouting distance of Buckingham Palace ... but most Londoners have probably never heard of it, and would be challenged to direct you to its backstreet entrance.

This is the only aristocratic townhouse in London that appears as it would have in the 18th century. Once, every noble family maintained these urban palaces, from which they ran business, controlled the social scene and pulled political strings. By the 20th century, however, the world had changed and cash-strapped old families were re-trenching to their country estates to try to survive. Many properties met the wrecker's ball, with whole rooms often being sold to museums or rich business magnates in the States. Though many of the buildings remain, most of them have now been converted to private clubs, offices or embassies. Spencer House is unique.

It wasn't always this way. Like so many others, it had been used as offices from World War II until the mid-'80s. Enter the hero of our story: Jacob, 4th Baron Rothschild. Businessman, philanthropist, passionate about art and history. (I dined with him at a business event in the '00s and had to fight to avoid fan-girl swooning.) Rothschild procured a 96-year lease from the Spencer family, who still own the place, and set to work. The walls and ceilings had been covered by modern plasterboard and were mostly intact. The restoration team cleaned, refurbished, re-gilt ... then set to work buying period furnishings to bring the place to life. So though the original residents John and Georgiana Spencer wouldn't claim any of the furnishings as their own, they certainly would have found them familiar.

A guided tour, only possible on Sundays, will take you through 11 rooms now glowing with 18th-century authenticity. You'll find all of the beautiful details you expect from any Georgian stately home: portraits and Old Master paintings, furniture in the style of Chippendale and Robert Adam, piles of worthy books, busts of great men, sumptuous draperies. What's different about Spencer House? First, because Rothschild selected all the furnishings and still uses the place for work, socialising and special events during the week, it has a "lived in" feel about it despite its Georgian credentials. Second, it has two rooms that are beguilingly gorgeous, and unique in British architecture.

The first is the Palm Room, one of the few interiors in Britain where neo-classical Palladianism approaches the giddy heights of Rococo. Its bones are pure Georgian: Lofty rectangle, classical frieze, column-framed fireplace, alcove on one end through a screen of columns. The flight of fancy comes with the columns themselves, which have been transformed into palm trees by riots of gold-gilt plaster. You get the idea that the plasterers had never actually seen a palm tree, but their loose interpretation only adds to the charm. At the centre of the alcove stands a copy of a famous Roman statue of Venus, beneath a dome and framed by three alcove-topping half domes all coffered with more gold gilt. One wonders if a young Prince George, someday to become the Prince Regent and then the 4th king of his name, found inspiration here that led to his madcap house at Brighton.

The other masterpiece is immediately above. While the Palm Room was intended as an after-dinner lounge for the gentlemen, couples came back together in the Painted Room. Designer James "Athenian" Stuart made the decorative scheme a celebration of marriage in honour of the owners' famously happy one. Several legendary marriages are depicted and deities of love dance across painted walls that would look at home in Pompeii or Herculaneum. Light floods in from the windows in the semi-circular apse, making the gilt details and dazzling white columns shine. How magical it would be to grab a book, stretch out on the green silk sofa between the mastiff-sized golden griffins that form the arms, and spend the afternoon here. But unless your name is Rothschild, that's unlikely.

If you are a female visitor, don't miss the chance to visit the ladies' loo before you leave. You'll find a fantastic collection of 18th century satirical cartoons and a 1/12 scale model of the house above that gives you another perspective on the decorative detail. The model by husband and wife team Kevin Mulvany and Susan Rogers took two years to make and is correct in every detail, from its hand-woven rugs to the furniture and paintings. It's as fascinating as any of the interiors upstairs.

Spencer House is open every Sunday from 10 am to 4:30 pm. You must go through with a guide, but that's a real treat. Unlike the well-meaning but often ill-informed volunteers who add little to a National Trust house, these are professionals hired by the Rothschilds to bring the house to life. They'll cover the history, the art, the architecture, the romantic story of John and Georgiana Spencer (ancestors of both Georgiana Duchess of Devonshire and Princess Diana) and how the decorative schemes throughout the house celebrate their relationship. It's a bargain at £15, and you get a discount if you're a member of the Tate, V&A, Royal Academy or the Art Fund. If you want a free sample, Spencer House is open to all during London Open House weekend ... usually in mid-September ... but the queues are prodigious and the tours are much shorter.

Give yourself a winter treat and pay for the full experience. You won't regret it.