Woah, that's quite a story. Must have been a gruelling 17 months and with no ZS! What are the 3 reasons they gave for faulting the garage? I can't see a cam belt failing so soon after being fitted unless something was fitted wrong, even if it was a cheapo belt.
Interestingly, over here when I needed a head-gasket changed, the garage I went to didn't want (or didn't know how?) to source the parts. They specifically asked me to get my own parts and as long as they were genuine MGR, would guarantee the work. Still charged a bomb and bet I'd have the same battle if something did go wrong.
Glad you have your pride and joy back