I live in Gloucester....my ZT-T V6 belts were done by Dave Allkins at MG Rover Mobile Mechanics in Derby.
http://www.mg-rovermobilemechanics.com/ They do mobile too but they're extremely popular so expect to wait for a couple of months for visiting their workshop or possibly longer for a home visit. That's who I would go back to for my belts - worth the trip!
Others I use for different work but also do belts are Chris Marshall in Burton Upon Trent (Marshall Performance Motors) and Tom Force in Leominster (Force Tuning). All are around £450 for V6 belts I believe.