Bank audit letters

We have a number of audit clients with tight reporting deadlines as they are subsidiaries of large groups. One of the major problems is bank audit letters. The banks often take months to respond, if at all and when they do you find on the audit that they have got them wrong. You phone them up and when you actually get through they agree to reissue them which takes a further 6 weeks way past the reporting deadline? What to do? You can't not sign off?