I've not had chance to look very far for a number for GMAC, so here's what I got:

Registered office:
General Motors Acceptance Corporation (UK) plc
Wesley House
19 Chapel Street
Luton
LU1 2SE

Their NGN is 0845 712 3444

This page gives their fax number as 01582 419617 and gives another NGN of 08705 464 622.

I have finance on a Vauxhall (suprisingly ), so that's my reason for contacting them. Any information would be appreciated.