Colin
It sounds like you have an airlock in the cooling system, try filling the system via the hoses going to the heater. Later systems had a filler in the hose going to the inlet manifold which was a great idea. However, Rover in their infinite wisdom have deleted this in their replacement hoses. Failing this it could still be the head gasket, in which case you will have to remove the head and get it skimmed (Haynes manual is invaluable here) and don't skimp on anything.
Good luck
come back if you need me !
Terence Wright
May 2006