I cannot speak for US law, but in the UK a cheque is only valid authority to debit your account if it is signed by you. A forgery is not signed by you. Different issues may be involved with a cheque guarantee card, but you do not mention one being taken as well. Also, a bank has a duty to its customers and I would suggest that the bank has been negligent in not detecting the forgeries. There are some problems about the Banking Acts in the UK - a bank may not be liable for wrongly paying a cheque that is "complete and regular on the face of it" (and "face" includes back). Here, I would say to sue the bank if all else failed. The "all else"! would be the bank's complaints system followed by a complaint to the banking ombudsman.

Also, assuming the thief is prosecuted, you could apply for a criminal compensation order against them.

Naturally you can sue the thief too - but she may not have any money.