what i was thinking is getting the lowest temp thermostat and one that opens a lot more than the one i have, when i tested the one i have in there already i heated up some water in a kettle and it opened up slightly although i didn't have a thermometer. now that you brought that plug to my attention i might give it a go.
i have drained out my radiator and i screw off the top hose and ran water through until it was clear, which was straight away as my radiator is has been recod with a new top tank, and new water pump and thermostat was installed not long ago , ran the car for nearly 45min and switched the temp control inside to hot until all air bubbles were gone should this do the same thing as the plug.
I've asked everyone and no one has this problem before although everyone says that on hot days it should go over alittle when running air con.
Thanks for your help guys much appreciated.