Indeed Tommygards, I suspected that after watching several videos on cars overheating issues, how this hand held pressure pumping device comes with different radiator caps and you are indeed right the device might seal OK but when you try an actual cap it may not be sealing properly.Glad you got it sorted 👍 not done one recently but ironically pressure testing is normally done by removing the coolant reservoir cap and fitting a different one that comes with the pressure kit that allows you to apply pressure via the cap…so even a pressure test may not have identified a poorly sealed cap.
In which case it is always better to remove one of the smaller pipes from the bottle and have one end of the pipe blocked with a purpose made plug clamped tightly, and make sure it points away from anyone, as the plug may shoot out if not tightly clamped, and pump air through the bottle spout where this pipe gets connected.
So far I have now been driving the car daily and not a drop of coolant has gone down. This is just with a loads of PTFE tape wrapped around the bottom seal of the cap. New bottle has arrived with a new Black Colour Genuine cap, it says 20PSI, as well as I ordered a new thermostat complete with new housing and a seal, The yellow after market caps do not carry any markings or information regarding what PSI they are rated at.