Spanish bank Santander is leading the race to snap up Royal Bank of Scotland's (RBS) 320 English branches for as much as £4bn.The Williams & Glyn's branch network is being sold for £1bn, but a potential buyer would also have to find an extra £3bn, according to a report in today's Times.Emergency funding provided by the Bank of England is currently keeping the business going, so the successful bidder must replace that within weeks.It must also find about £2bn in capital to support the £24bn loan book.Santander, which already owns Abbey, Alliance & Leicester and the savings arm of Bradford & Bingley, has deep enough pockets to finance the acquisition, as does Virgin, say experts.Spain's second largest bank BBVA is also thought to be in the running, but National Australia Bank may be having second thoughts, according to reports.RBS was ordered to sell the branches by the European Commission as punishment for accepting state aid.