Pull down the service manual(s) for your truck (Google something like "defender workshop manual"). They are free and you should have it if you're going to own something that old.
I'm assuming your gauge is not working, and that you've confirmed your engine is actually heating up (i.e. thermostat is OK).
Locate the temperature sending unit. Open the dash so you can get to the temperature gauge wiring. You should be able to figure out what's wrong in just a few steps:
1) Pull the wire off the sending unit and pull the corresponding wire off the gauge. Measure the resistance. If the circuit is open, you have a bad wire.
2) With the sending unit wire still disconnected, run a wire from the gauge where the sending unit wire was attached to ground. Very briefly turn on the ignition and turn it off. If the gauge moved (likely pegged), then your gauge is probably OK.
3) Put everything back together, cleaning all connections and making sure all connections are good. See if your gauge works now. If so, probably a loose connector - replace/fix the connectors on the end of the wire.
4) If the above doesn't isolate problem, replace the sending unit.
"Every normal man must be tempted, at times, to spit on his hands, hoist the
black flag and start slitting throats." - H. L. Mencken