Britain's high street bank chiefs have been summoned to the chancellor's office today to explain their failure to re-start lending to small businesses despite receiving £37bn of taxpayer's money.Heads of Lloyds and Royal Bank of Scotland - the two state-controlled high street banks- will be joined by bosses from HSBC and Barclays and asked to justify their lending policies.Alistair Darling, the chancellor, is reported to be "extremely concerned" by rates on business loans rising sharply when base rates are at an all -time low of 0.5%."That is why we will be going through with each individual bank asking them why is it, at a time when the cost of borrowing is coming down, it would appear that the cost to small business appears to have gone up?""We're playing our part, the banks have got to understand that the public will not understand it if they do not play their part to the full," he said in a interview.