Rover 214 si 98 temperature gauge not working?

Could easily be the sender unit in coolant elbow - the one with a single wire. Take the connector off, and earth the plug inside to the cylinder head (I used a jeweller's screwdriver) and turn the ignition on. It the gauge jumps to the top of the Hot section of the dial, the sensor needs replacing. If not (and you're sure you earthed it), you need to check the wire in the instrument panel for continuity. If found, the gauge itself is faulty, if not there's a break in the wire somewhere...