I've just read an old post by Zeke where he was experiencing the same symptoms ... high temperature readings but no actual water loss or blowing radiator, turns out the temperature senders can exhibit strange resistance characteristics before packing up altogether.

I think I'll go for the cheap possibilities first so I've ordered a new temperature sender and a bottle of Water Wetter - a grand total of £25 - if nothing else this will confirm if I have a more serious issue to deal with before Prescott !

Thanks for the feedback guys