Bullock has been bought by Lloyds Development Capital for an estimated £100m

It is understood that the Bank of Scotland, which funded a £42m management buyout in September 2005, sold the £140m-turnover company last weekend. Bullock employs 600 people and operates in the Midlands and north of England.

The sale coincides with the retirement of John Gaffney, the managing director, who left Bullock on Wednesday owing to ill-health.

This is the latest in a wave of mergers involving medium-sized firms and it is believed Bullock looked at a trade sale before opting for the private equity route.

One source said the deal had been on the table for six months and was tied up last weekend ahead of changes to Capital Gains Tax (CGT) laws on 6 April. The new rules mean partnerships and individuals now pay a flat rate of 18% on gains if they sell up, compared with the previous maximum of 10%.

Another source said the decision to sell had been driven by some of the senior management team wanting to depart before the CGT changes came into force. As Building went to press it was unclear which senior managers would stay, but it was understood that Paul Hooper-Keeley, the finance director, was likely to leave.

One source said the deal had been on the table for six months and was tied up last week.

One industry observer said the financial certainty of the social housing market made firms in the sector attractive to buyers in the current downturn. He said: “You’ve got a first-class revenue stream from the government and public bodies. In this environment there is no better guarantee. The other advantage for private equity buyers is cash flow, in that social housing contractors are paid ahead of their own outgoings.”

Bullock was formed after the Bank of Scotland-backed buyout of Montpellier’s social housing division in September 2005. Montpellier now trades as Renew.

In February Bullock won a place on two frameworks that will deliver a combined £500m of new social housing over four years.

Bullock would not comment on the sale speculation.