Not an expert, but sounds like it is either the thermostat or air in the system. I thought there was a certain way to fill the system, which involved using the plastic bleed screw in one of the coolant pipes (next to the block, just below and forward from the throttle body) - maybe the garage you used have not bled it properly after refilling? Might be worth finding the filling procedure, and draining the system and refilling it properly. Wouldn't take long to do.
With regards Mg specialists, sorry, don't know of any personally in the area. When I had my belts and water pump changed, I had the guy from Derby come down (just outside of Kettering). MG Rover Mobile Specialists or something like that. Another possible is a guy over in Olney. Phil-T4 on the forums. Although a small job, he did my key fobs a few years ago, and was saying he does work on Kv6's although at the time was more ZT, I think. Might be worth a call? Also called MGR T4 Services.