Thanks for the reply. I did some running around to a few auto parts stores today and I have ordered a temperature sender which is not Rootes, but may suit. I will measure my current sender and compare it with the new sender once in my hot little hand (early new year). The only other issue is that the thread is 1/8 BSP where my current sender is 3/8 BSP (that is not an issue).
I will let you know if it works out OK.

Merry Christmas, Robin.