If your ISA transfer takes more than 30 days then write a complaint first to the bank in question (Head Office unless you know the exact department). I would also advise that you seek compensation for any interest that hasn't been credited to your account.
If that doesn't succeed then make a formal complaint to the Financial Ombudsman Service (see link below). They will investigate your case and often seek compensation as well, the average being £50 - £100.
If you do complain to the FOS then the more supporting paperwork you have the better. The complaints procedure is relatively simple and all the information is on their website.