Corporate bank executives are continuing to stretch lending margins for many customers. In interviews with the Financial Review, executives at Westpac and National Australia Bank estimated the increase in margins at between 30 per cent and 40 per cent (of the prior margin over the benchmark, such as the bank bill rate) for all borrowers.NAB's executive general manager of business and private banking, Joseph Healy, estimated that margins had doubled for borrowers in those sectors most exposed to the slowdown such as retail, leisure, hospitality and some areas of transport; basically any sector dependent on discretionary consumer expenditure.