Can anybody suggest what my problem might be?

On the dash of My 2000 Reg Golf the engine temperature is always displayed as zero, the dial does not move up to about 90 as it would normally. This has been happening on and off for about a year, every time I think it is fixed, it stops working again.

When the problem first occurred the dial would move as the engine warmed and then flick up and down day to day. Initially I thought it was the thermostat, so I replaced it using a help guide I found on-line and it started working.

After a few months of working the dial stopped working again, this time it would either work or not at all. This time I replaced the Coolant Temp Sensor (the green plastic one with the O ring), again for a few months it worked fine and then stopped working one day and hasn't since. I have been driving around for a few months with the engine temp reading as zero, the engine gets hot and it doesn't seem to perform any differently. Last weekend I had another look at it, replaced the thermostat again (as they are easy to get hold of) but no change, after a long run the engine is hot but I don't think the radiator is, I could put my hand on it after about 60 - 70 miles driving.

Could it be that there is another sensor in the radiator and for some reason the water is not getting to it? Should I replace the green sensor again?

Details of the car are: 2000 VW Golf 1.6 16 valve engine code AUS (I think).

Any suggestion of things to try would be very much appreciated, I really don't want to spend £100 putting it in a garage.

Thanks