By Jack Mendel London-listed software firm Gresham Technologies has been bought out for just under £150m by an American private equity firm. Announcing the move to markets this morning, Gresham will be purchased by Alliance Bidco, a company owned indirectly by funds managed by STG Partners, a US PE firm. It will combine Gresham with another of its portfolio firms, Alveo. Gresham’s board said that the “low level of liquidity” on the London Stock Exchange was one of the deciding factors for giving up its public listing – a now-regular refrain from small and mid-cap listed firms. Eachshareholder …