Review: Houghton Hall, the Prime Minister, the Empress and the Heritage Kenwood House, London

Sir Robert Walpole (1676-1745) was the first British politician to take the title Prime Minister and, next to the Kings George I and George II, was probably the most powerful man in 18th-century England. He was a big man in every way: large in ambition, huge in achievement and colossal in the flesh. According to Lady Montagu, his stomach entered a room "at least a yard before his nose".

When not controlling the affairs of state, which he did for a total of 23 years, Walpole played the role of country squire: a little bluff; a little bawdy; fond of a good feast, a crude joke and the company of his fellow huntsmen at Houghton, his Norfolk estate. Yet, for all the mud on his breeches, he was also a man of considerable taste who amassed a fine collection - a selection of which has been gathered from around the globe and is currently on show at Kenwood House on the edge of Hampstead Heath.

The exhibition opens with a group of portraits of Walpole by Kneller and Wooton, the English painters who did best from his patronage, and a large seated version by Stephen Slaughter, a minor artist better known for his role as keeper of the King's pictures. It is an unremarkable bit of painting but, even so, Walpole's 20-stone bulk clad in the robes of Chancellor of the Exchequer ensured that it was a commanding image: huge feet and hands; pink cheeked and rather grand. It was painted in 1742, shortly before Walpole's resignation was finally accepted by George II - a resignation refused for years despite gathering scandal and rumours of his inclination to dip into the public purse.

Despite these tendencies he still died with debts of pounds 20,000 and so ensured the inevitable sale of the collection he had spent so long creating. As his son Horace moaned, "Houghton, I know not what to call it, a monument of grandeur or ruin."

In 1779, 181 paintings were sold to Catherine the Great for pounds 45,455, a valuation set by James Christie (later of auction house fame) and the current exhibition is the first attempt to reunite some of them with the furniture and objects that still belong to the family. It is a show with several good intentions: first, there is the story of Sir Robert himself; second, that of Houghton the House; and third, the contents of the collection and tragedy of treasures lost to the nation. The first two are well enough served: there is a clear sense of the man and a palpable sense of the place, but the final and most important part of the equation is a bit of a disappointment.

It starts well enough with the portraits I have mentioned and leads to a selection of paintings and objects from Houghton's family rooms and good plain furniture in a sort of restrained 18th-century style that wouldn't have upset Walpole's Norfolk cronies. These are set alongside portraits of his wife, children and gardener and complemented by some striking if less personal pictures, such as Salvator Rosa's Portrait of a Man and Kneller's portrait of the Spanish poet Joseph Carreras - one of the best things in the show. These quiet pleasures occupy the first half of the exhibition, but as one turns to Houghton's public rooms, the point at which the exhibition should take off, the momentum is lost.

As one would expect, the furniture steps up a gear to the carved and gilded exuberance of William Kent's designs, but the selection of paintings doesn't measure up to the task. Walpole owned at least eight paintings by Poussin, including a couple of masterpieces, but he is represented here by the rather leaden Holy Family. Other disappointments include a "Velasquez" portrait of Pope Innocent X that is probably by someone else, and a portrait by Frans Hals that now belongs to the National Gallery of Art in Washington DC. It might look all right back there but, at Kenwood, one is too aware that there's a much better one housed in the Iveagh Bequest just a few rooms away.

To 20 April. Kenwood House, London NW3 (0181-348 1286) Further arts reviews, page 11