The temperature sensor is probably dead or the wiring has melted. Or the connection is corroded. Or the motor has burned out.

It's a Rover. You don't expect durability , surely?

No idea of Rover layouts - sorry..

You really need a simple multimeter and some knowledge to sort out the problem....

I know what it feels like you hope for a simple answer but I think in this case its a strip down.The above mail is very accurate ,for future knowledge the main ecu sits under the wiper housing on the left hand side under the plastic trim in front of the windscreen these often fry due to the water drain getting blocked but in your case the odds are its the head gasget.I have had exactly the same problem on a 75 and a Freelander a friend in Bradford specialises in Rovers and he must do one a week once they are repaired properly they never go again and I repeat properly.

The other main problem on the 75 petrol is the fuel pump situated under the rear seat its in two parts and one of the parts has the habit of seperating there is a mod available for this or you can fit a PSA unit its worth doing as if it goes its a real messy problem to sort out, I suggest you search on the Rover 75 forums for further knowledge and help.And good luck.

Showing the forum messages to garage had magical effect.Sender contacts were examined,diagnostic m/c coupled and cooling fan DID kick in at 102/103C. Saved unnecessary job,so inlet manifold O rings next!!