Reply to "Some good...some bad! It passed the MoT test ... But it over heated on the way home."

a dogy temp gauge is possible. DO you know where your temp sender is located. there are at least three possible locations. the original location is the Supply (swirl) tank. the better location is just below the stat and some have used the water pump aux return plug. using a test thermometer is the best way to determine the actual temp to compare to the gauge. A lot have purchased those non contact IR guns

the illustration below is the original, the arrangement can be modified though. is your expansion tank hose arrangement like shown? or is there a third lower hose back to the water pump.

what type of caps do you have on the tanks. the cap on the supply (swirl) should be the pressure cap, but the cap on the expansion (recovery) is just a cover so it allows overflow to dump to ground

why are you saying "boiled over"?