First Time at The Box by Alfred Reginald Thomson (1894-1979) – estimated at £3000-5000 at Sworders on October 4.

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The unfinished and unsigned painting depicts Britain’s first female prime minister at the dispatch box on her first day in the job on May 4, 1979.

Hadden bought it at Sunbury Antiques Market’s Kempton fair in 2015 from a trader who had bought it in a London house clearance.

However, establishing who had depicted Margaret Thatcher on her first day took quite some time and effort.

Hadden, who has form with selling Thatcher related items (he sold a group of her dresses at Christie’s in 2012), says: “I knew it was special. It is really large and I knew it must have been painted by someone who knew the House of Commons. I started to research who it might be and emailed the Conservative party’s historian Lord Lexden. He invited me up and I showed it to him and he was extremely helpful.”

With the help of Lexden and Lord Cormack, a member of the Commons Works of Art Committee through the Thatcher years, it was established that the artist was likely to be Alfred Reginald Thomson (1894-1979).

Thomson had been commissioned to paint the Commons in 1960 with Harold Macmillan at the despatch box. He was given permission to sketch from the public gallery in the chamber and then for six months did individual portrait sittings with MPs. Thomson is also the only artist to produce pictures of both Houses in session (he painted the Lords in 1961-62). But there was no record of a commission for him in 1979.

Thomson Pic

Alfred Reginald Thomson (1894-1979).

Although housebound by 1979 (the year he died) Thomson was averaging 15 small pictures a week. On hearing the news of Thatcher coming to power he decided to recreate his Commons picture from sketches and memory. To help confirm this, relatives of Thomson were sought.

Hadden adds: “I needed to track down surviving relatives but there was no record of any with the collecting agencies so I tracked down where he had lived (Fernshaw Road, Chelsea) and put leaflets through every door.”

Luckily one neighbour responded with information and grandson Paul Mannings was tracked down to Edinburgh with the help of Artists’ Collecting Society (ACS).

Mannings had sketches from Thomson including those made of the Commons. With this evidence, and other information, the picture was finally authenticated by Malcolm Hay, the then curator of the Parliamentary Art Collection.

Now Hadden has put the picture up for sale at Sworders next month. It carries an estimate of £3000-5000 and has been on display at Sworders’ Cecil Court gallery ahead of the auction on October 4.