The reason I asked about the location of the stat is to be certain that you were aware of the two different types of stat and their different locations that Rover used on the 1.8 variants. Some have not been aware and have ended up with two stats fitted as a result, a PRT at the radiator and a standard traditional stat under the inlet manifold. Only one should be fitted and when two are unwittingly fitted it does tend to lead to problems. There could be a number of causes and given that the car is a fairly recent acquisition and it came to you with previous HGF work having been done I wouldn't rule out any of them just yet - two stats included. The first logical step should be to ensure that the cooling system is properly configured, I wouldn't want to check for two stats just yet but I would check that the prt stat has been connected correctly as a mistake with this is not unknown. Link to a diagram of how it should be.
Click