Spanish bank Santander is expanding its overseas empire with the acquisition of 318 branches being sold by government-owned Royal Bank of Scotland (RBS).The sale, thought likely to make about £2bn, is part of a deal agreed with the European Commission as RBS's punishment for borrowing billions from the British government.Edinburgh-based RBS is offloading the former Williams & Glyn's network, a brand widely expected to be dropped by the Spanish.A final price for the deal, which could take 18 months to complete, won't be known until next year.Santander has already bought and rebranded Abbey National, Alliance & Leicester and Bradford & Bingley's savings and branch network.Once the transaction goes through, it will run over 1,600 branches, more than HSBC with less than 1,400, but behind Lloyds, Barclays and RBS.