It's in the bottom of the inlet manifold, where it's almost impossible to reach. There. short and sweet ;-)
Note: there is a sensor for the temperature gauge and MAYBE another for the ECU coolant temperature sensor? MORE INFO PLEASE
Coolant Temperature Sensor to ECU, Single Injection Coopers FAILURE/COLD RUNNING
If the temperature drops way low when engine is hot and the car dies or runs like it is on CHOKE suspect a failing or failed engine coolant temperature sensor, mounted on the inlet manifold there are issues with both the sensor failing and baring out of the associated wires to the ECU. A fult diagnositc download by your ROVER dealer may identify this problem. Engine appears to run fine after resting for a few minutes. ( added by FB 27 April 2003 from PERSONAL experience)
IMPORTANT: generally electrical senders and gauges are commonly MATCHED PAIRS: unmatched pairs may over or under read in combination
If you try to pair up sensors and gauges that were not originally used together you may get a gauge that over or under reads!
You may be able to check a replacement sensor/gauge from its temperature/voltage/resistance characterisitics versus the WORKING old one.
Capillary/Mechanical gauges linked from a bulb by a fluid/gas filled tube may equally have different gearing BUT should response the same if the bulb is replaced so long as the liquid or gas is IDENTICAL in volume and resting pressure. NEED TO VERIFY THIS.