Noticed the car overheating a little popped the bonnet an there coolant bubbling in the overflow bottle an it's full to the brim I have a rad cap with a pressure realease on it could that just be the problem or is it something more serious?
maybe thermostat is fucked or the rad is blocked or something
Sounds like a headgasket unfourtunatly,
mind you its worth draining everything and refilling it as if its right to the brim u might have to much water in ur system and is causing the back pressure!
When u say bubling is it just bubbling, or is the water boiling over??
Well if it wasnt overheating before and then u change the thermo and it overheats I wouldnt be looking past the thermo for now!
Dont ignore the most obvious prob, sure bang the old one in and see what happens!