Nick, I hope you're recovering well.
I agree with landsman and would bleed the cooling system before looking to blame something unrelated.
Here's some bleeding advice:
Of interest, early M52 motors, which I think you have (and the earlier M50) had water pumps with plastic impellers which are known to break up or spin on the shaft. However, as you've disconnected the heater circuit I'd look there first before worrying about the pump.
The following is copied from Bavarian Autos website - they sell a pump with a steel impeller:
"The earlier versions of these engine families (M50 & M52) were produced with water pumps that had “plastic” (actually, various types of resin and plastic compounds) impellers. These impellers have a history of cracking at the shaft hub and slipping, or actually breaking apart and disintegrating. Either way, you would end up with a water pump that looks fine on the outside (no leakage and no shaft play), but will not move coolant through the engine and the radiator. Symptoms for a cracked and slipping hub would be that the engine may run at an appropriate temperature at idle speeds or slow driving speeds, but will overheat if driven faster (higher engine speed and more heat produced). Symptoms for a broken apart impeller would be that the engine temperature just continues to climb, no matter what the engine speed is …. just as if the drive belt were off of the water pump and the pump was not turning."
I think the important symptom for you is a cold heater which usually points to an airlock or low coolant level.
Looking forward to your performance testing when this is sorted :-)