Hi
Exact same thing happened with me, found out that a change of correspondence address letter had been sent to them with "my" signature on it (it's very clearly not my signature). I had to put in a subject access request to the bank to find this out (they send you a copy of everything they have on file for your mortgage account).
Doesn't really mean anything though, I thought this might help me in some way as a kind of proof that I didn't know the house was in arrears but the attitude is (and I can't argue with it in fairness) that it's up to me to keep an eye on my affairs.I still think it's insane that my account got to over 24K in arrears, the bank was moving to reposess and I never got so much as a phonecall.
Maybe it would be worth checking to see if that did happen for you too, as I say it hasn't helped me but maybe it will in your case, especially if you want to appeal with the ICB?