The combination of Abbey and A&L will give Santander about 14% of the UK mortgage market, making it Britain's second-largest lender behind the Halifax.
Emilio Botin, chairman of Santander, said the deal was a "significant step" for the Spanish firm in the UK. It has echoes of its takeover of Abbey, which happened after the High Street bank ran into difficulties of its own.
A&L has been rocked by the carnage in financial markets around the world and by the meltdown of the mortgage market in the UK.
It has also seen board upheaval this year with Bennett forced to take time off ill. Last week it appointed Alan Gillespie as its permanent new chairman following the death of previous incumbent Sir Derek Higgs in April.
The deal with Santander was seen as a major vote of confidence for the British banking system after one of its worst periods. The collapse of Northern Rock - which included the first run on a British bank since Victorian times and subsequent nationalisation by the Government - seriously damaged the reputation of the City.