I wonder about that brace of Titians. The Duke of Sutherland is graciously letting Britain have first dibs on buying them for £50 million apiece. Sundry art experts have been wheeled out to assure us that the paintings are worth £150 million each and that the British public will be getting them at a knockdown price.

How on earth do they know? Titian masterpieces are not like Marks & Spencer shares or Surbiton semis; there is no deep, liquid market in them. The potential buyers for Diana and Actaeon and Diana and Callisto can number no more than a few dozen billionaires.

The most that any Titian has ever fetched at auction is £7.48 million — in 1991 for Venus and Adonis. The last time that a Titian came on the market was in 2005, when Portrait of A Lady and Her Daughter failed to reach the £5 million reserve price.

Only five paintings have ever fetched more than £50 million — two Picassos, a Klimt, a Willem de Kooning and a Jackson Pollock, all works of relatively recent artists. The fact is that the only private individuals able to afford these things — hedge fund managers and oligarchs - prefer modern art.

No painting has ever reached anything like £150 million. The record is $140 million (£77 million) for a Pollock in 2006.

So how can we be certain that £100 million is the “stupendous deal” that the National Galleries of Scotland assure us that the public will be getting if they manage to scrape together the cash?

Financial value has little to do with the intrinsic merit. Titian's profound influence on Constable may carry less weight with buyers than whether the canvas fits in their Manhattan apartments.

The only financial value that can be put on a painting is what someone somewhere is prepared to pay. Insurance valuations are meaningless.

The best people to judge the price of these paintings are not auctioneers and museum curators, but the advisers to the world's plutocrats - private bankers and family offices. They know the real appetite for these things. And the real appetite may have less to do with aesthetics and more with the kudos associated with expected capital gain.

It may be that art, along with vintage wine and philately, is in the last phase of a prolonged bubble and is soon to burst. With shares, property and commodities sliding, how much longer before these non-yielding baubles follow?

The National Galleries should shelve their fundraising efforts in Britain and head for the Gulf and the Far East. A sovereign wealth fund, needful of a dose of positive PR, might well be persuaded to snap up the pictures and put them on permanent loan to Britain. A case could be made for them to diversify their portfolios, surely a little too overweight in Western banks these days?

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