I have an elderly, but nice, Pace aluminium frame with a crack in the headtube that I’d like to get repaired. Does anyone know of a good quality fabrication / welding shop in the West Midlands area that might be able to help ?

I have no idea what the timescale would be, or even if he could fit you in, but Craig Taylor at Dyno Torque in Brum has done a lot of TIG jobs for me many moons ago, and bloody good he is too.
He’s a very very busy man, but he’s also an absolute legend, so he may be able to do something for you.

I looked at a pace frame a few years back with a crack longitudinal down the center of the head tube. It wasn’t repairable Safely in my opinion. But that depends on how much you value your teeth.