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"I have been meaning to ask you for some time whether you would care to paint a family picture of myself and our two daughters. And if you could do this, might I ask what sort of price it would be? I do not like to mention prices etc., but I would like to pay for it, and if you would paint us all, it would be a delicious picture I know. Please let me know, and do not hesitate one moment if you feel you would rather not embark on a family group."

The 'grand manner' oil portrait that Brock produced - an informal and endearing scene of blue blood childhood bliss - was titled simply Portrait of Her Majesty the Queen and her children, Their Royal Highnesses Princess Elizabeth and Princess Margaret.

But did it or a copy ever enter the Royal collection?

Certainly it was among the exhibits at a display of Brock's paintings at the New Burlington Galleries shortly before his death. But its subsequent whereabouts were unknown until 1980, when the then owner - Douglas Wedderspoon, a descendant of Brock's lover - offered to loan a version of the painting for a gala evening at the Royal Opera House in Covent Garden attended by the Royal Family. After the event Mr Wedderspoon received a letter from The Friends of Covent Garden, thanking him for the gesture. "This [the painting] was much admired by the Royal family, but you may be interested to know that neither the Queen nor Princess Margaret knew where the original is now to be seen. I did not have a chance to ask the Queen Mother herself, but the Queen and Princess Margaret both remember very well going for the sittings with Mr Brock, particularly I think because of his being so deaf, and messages having to be passed to him by sign language."

The portrait, measuring 4ft 4in x 3ft 4in (1.32 x 1.01m), was consigned for sale at the Fernhurst auction rooms of John Nicholson on March 24 by a relative of Mr Wedderspoon. Sold together with the Queen Mother's letter discussing its commission and other associated literature, the estimate was £20,000-30,000.

With several commission bids on the book, and a telephone bidder, the painting was eventually secured for £24,000 (plus 15 per cent buyer's premium) by a local private collector.

Gemma Cross Brown