Couple of guys on here print vinyl stuff- they might have something.
At last I bring some good news! Well, in a way.
This evening we took the water pump out to find the impeller inside wasn't attatched to the shaft at all, meaning no water was being pushed around the engine once up to temperature, causing the overheating and thermostat/blocked rad type symptoms.
I'm possibly the only person to be genuinely pleased that my water pump is knackered.
Also took delivery of some nice silicon pipe work off another GTB that was being broken, so my engine will not only work, but start to look quite good too