This is on a 2000 1.8 petrol weekend, but I think the stat is common on 1.4 1.6 and 1.8. I have noticed the car was slow to warm up a couple of times, then this week on the way home at 50-ish (speed restricion on the M way) the gauge started to climb. I put the heater on full and it helped a bit keeping it 1 calibration mark away from the red, once or twice it dropped back to 1 above 1/2 way( usually "painted on the gauge" at 1/2 way) but crept up again. I kept going as I don't like stopping on motorways, when I turned off onto the next motorway with no restriction as the car settled at about 65 in the traffic there the gauge suddenly went back to normal in about 1 second. This could be sender faulty or sticky stat, anyone else seen this, or the same fault with a different cure? The car has been fine since but I got a new stat jubilee clips and antifreeze and will swap it out.

Also have a very close look at the rad.

Done it, what a awkward job! Few points;
Does anyone know a quick easy way to get those cheap and nasty one time only hose clamps off that the car comes with?
The Haynes manual says remove the battery, not needed but you do need a unversal joint and extender for the 13mm socket.
The manual also tells you to bleed the heater using the screws near the bulkhead in the hoses, err my car doesnt have them, and the heater works fine anyway.