[Brian]

Cool! Right, that is the ideal location. Seems to me that if radiator was pressurising, the tank would want to expand outwards, from your description, it sounds as though vacuum is sucking it inwards. I cannot comprehend this. As you understand, the outlet /overflow is open to atmospheric pressure, even if it is running into catchcan as I suggested you fit, and which you have done without success. Therefore the system can 'breathe'. Can you try testing again, but leave the radiator cap loose? , ie, not screwed down tight. See if it behaves itself like that.