This is something I would recommend as the radiator caps have been upgraded.in recent years, but I would buy both, I tried a new old style cap (yellow) and the new version genuine Vauxhall one (black) and whilst both worked for.just felt the old OEM yellow worked better for my vehicleSomething is causing pressure to build in the system and as you have ruled out a stuck thermostat I would try a new cap as I have read they can give problems releasing at too low pressure if that doesn't fix it I would get a pressure test done before spending any more money.
Have you done a compression check on the engine? I experienced something similar to this in the past which turned out to be a leak between one of the cylinders and a coolant gallery between the block and cylinder head. In my case it was a failed head gasket. If you can see any bubbles coming into the expansion tank when you rev the engine it is highly likely your head gasket is leaking.I am experiencing a build up of pressure resulting in water being forced out expansion tank vent, radiator pipe at passenger side is hot , but the pipe at drivers side is cold any advice welcome
I hope i am wrong but the fact the thermostat was stuck open (as you said you changed it and the pipe was then hot) and that you now have gouges that would let pressure out through the cap makes me wonder if the seller was hiding the beginning signs of a problem that has now got much worse, i cannot think of any other reason someone would do that to the expansion tank.Had car about 4 months, it must have been like that when we got it as it wasn’t us that drilled it , anyone know why someone would do that and for what reason?