Sigma revealed yesterday that Sir Tom's vehicle West Coast Capital had cut its stake from 6.19% to 2.26% last Thursday, after the AIM-listed group unveiled a deal endorsed by David Cameron and Vince Cable which saw its shares jump from 26.25p to 37.75p.
At the same time Henderson Global Investors reported it had taken its stake to 11.76%. The London-based investment house, however, had picked up most of it on October 18 when the shares were at 22.12p, acquiring a 9.95% stake.
Sir Tom first backed Sigma in 2007 as one of a clutch of high-profile backers, including Vincent Tchenguiz, Scottish & Southern Energy and Bank of Scotland, of a sustainable energy investment fund.
But at the end of that year Sigma issued a profits warning as both its property and investment businesses began to suffer from the downturn.
Sigma's shares, then trading at 38.5p similar to now, plunged to 5.5p in early 2009, but rallied to 17p later in the year as the group successfully spun out Frontier, its university IP commercialisation subsidiary. In August 2011, Sir Tom took his stake from 20% to 22% as part of a deal to sell to Sigma for £347,000 his Inpartnership business, enabling Sigma to reinvent itself as a regeneration specialist.
Graham Barnet, chief executive, said the high-profile deal with political backing had "the potential to transform Sigma's rate of growth and should help to establish Sigma as a major presence in the delivery of high quality new homes for the rental sector".
A spokesman for West Coast Capital said: "We have been selling our stake down in Sigma as part of our investment strategy. In this case, having written the value down to its all-time low on our books, we decided to take some profit. We do, however, remain committed to the business as a minority shareholder."
Henderson declined to comment.
Sigma's shares rose 1.25p to 36.25p, valuing it at £16.6m.