They will now count Dreweatts, Bloomsbury, Baldwin's and Apex alongside Mallett in their group, giving them high-profile brands that range more widely across the market as both dealers and auctioneers than almost anyone else.
Stanley Gibbons chairman Martin Bralsford said: "Stanley Gibbons is committed to delivering its established strategy to become a leading online collectibles marketplace and a global auction house for fine and decorative arts, collectibles and other valuables. We are delighted that the board of Mallett has recommended the Offer, a logical and affordable next step for us to acquire a valuable brand and relevant assets. The Mallett business will expand Stanley Gibbons's expertise into an adjacent collectibles area, drive significant cross-selling opportunities across the combined businesses and build a stronger auction platform in the collectibles marketplace."
Mallett's non-executive chairman Lord Daresbury added: "The Mallett Board believes that Mallett's business would benefit from becoming part of a larger group with resources to support its development. In addition, the Offer provides Mallett Shareholders with the opportunity to realise their investment in cash for a fair price now."
The deal brings clarity to the future of one of the oldest and most esteemed dealing firms in Mayfair, which has struggled in recent years, having moved from Bond Street to the grand surroundings of Ely House in Dover Street, the former palace of the Bishops of Ely.
However, in April Mallett announced an increased turnover of 16% to £11.8m for 2013 (up from £10.2m in 2012) and a reported profit before tax of £500,000, an increase from 2012's pre-tax loss of £100,000.
Figures were mainly boosted by strong sales in the first half of the year for Mallett US and resulted in a sales total of £6.8m, the second highest from the US showroom since it opened in 2003 and over double that of 2012, which was £3.1m.
This led to Mallett announcing that they had reversed an earlier decision to close their US operation as part of a cost-cutting exercise and would keep it to "maximise potential in the US market".
Baldwin's are due to move into Stanley Gibbons' Strand headquarters in November following the £4.5m disposal of their nearby Adelphi Terrace premises as part of the group's strategy to rationalise their property portfolio and integrate operations. Meanwhile Dreweatts and Bloomsbury Auctions retain their London HQ in Maddox Street.