He was right: the house is still amazingly fresh today. Set amid good-mannered cream stucco villas and post-war redbrick neo-Georgian houses, it is nothing if not assertive, commanding its acre of gardens regally. It was built for Ernest Ridley Debenham, of department store fame – though he only took a lease on it, and the family quit the house in the early 1940s. The freehold is owned by Ilchester Estates, proprietor Charlotte Townshend - a very wealthy London and country landowner who is a fixture on the Sunday Times Rich List. For the first time, Ilchester Estates is offering to sell the freehold of the house. For the past three decades it has been used as offices and meeting rooms for the Richmond Fellowship, a housing and employment charity which will soon be moving out. They have looked after it well: apart from dividing up a couple of big rooms for offices, they have kept the interior intact in all its weird splendour.

It is oddly arranged by modern standards. It is set back from the street behind a formal front garden - with a small back door in the middle of the façade. The main entrance is to one side – but you get to it via a covered way lined with de Morgan’s tiles, running parallel with a carriage drive. You pass through a little entrance vestibule, up some steps, and into the most sumptuous central hall you would ever expect to find in a London house. This domed space, like something out of St. Mark’s in Venice, is almost wholly lined with sparkling mosaic in blues, greens, reds, gold. What is not mosaic – columns, lintels, balustrades, fireplace – is made of a variety of marbles and other veined stones. The effect is layered, because corridors lined with de Morgan’s blue-green tiles run round the hall, to be glimpsed through the columns.

If you think the effect of colour is overwhelming, this is because Ricardo had not originally planned it quite this way: the lavish mosaic work was done for the Debenhams after they moved in and includes portraits of themselves and their children. What Ricardo thought of it, is not recorded. Being Byzantine, the motifs are more Lord-Leighton-classical than Pre-Raphaelite: but the mosaicist Gaetano Meo synchronised his palette of colours with Ricardo’s to good effect.

This powerful hall impacts hugely on the rest of the house. The rooms are all arranged around the edges of the plan, and are smaller than you might at first expect because so much of the house is taken up by that central space and the corridors running around it. As a result, the rooms feel the opposite of grand: cosy isn’t quite the word, but they are certainly on a domestic scale. The best rooms are the panelled dining room and the small, exquisite, library with its mother-of-pearl details to the bookcases. The big drawing room appears to have lost its original ceiling, since everywhere else they are marvellous. The ornamented ceilings are by Ernest Gimson, timber carvings by William Aumonier, enriched glazing for doors and windows by E.S.Prior, Ricardo himself designed the marblework and many of the other details. Even the plates for the light switches are finely wrought in brass with inlaid coloured glass. You might not know all these names, but take it from me: these men – each individually talented architects and designers - together comprised an Arts and Crafts supergroup.

The tilework extends - with a more austere pattern - to the huge kitchen in the below-stairs warren of rooms. The gardens out the back are nice in a rather unimaginative, formal way – made more interesting by a classical Ricardo garden pavilion, reached via another covered way from the house. Out to one side, down a flight of steps, is something quite different: a dank open box with curious stonework projections. If you can imagine a squash court from Gormenghast, that’s the general impression. It turns out to be an Eton Fives court (you clout the ball with big mittens, apparently), so presumably we now know where Lord Debenham went to school. There being not much call for Eton Fives among today’s wealthy housebuyers, Ilchester Estates has got planning permission to turn this area into a covered swimming pool, with a gym to one side and a private cinema behind.