I've had exactly the same symptoms recently so got the thermostat replaced. Decided against doing the job myself after reading up on it:

AudiWorld Forums: Thermostat replacement procedure..>>

Looks like this is a relatively common problem. I'm having my 'stat replaced by an expert in just over a week, will report back whether it cures the problem.

Nobody seems to have mentioned the obvious part here: does the heater warm up as normal or does it take a lot longer?

If it works normally, then it's the temp sender.

If it takes ages, it's the thermostat stuck open.

If the fan is kicking in before the temperature gauge reads "normal", then either the engine temperature sensor or the fan thermo-switch is faulty.

Finally got the thermostat replaced (by a skoda dealer) - engine now warms to 90 degrees just like in the good old days .

Thank you to all who commented on the problem - it was a big help knowing (roughly) what I was talking about when it came to verifying that the problem was indeed fixed.

Have taken the car on the usual family visiting trips of late and note that the fuel economy has also returned to pre-fault values. The running cool fault looks to have reduced the economy by about 4% to 5% (based on 100+ motorway miles at 60-70 mph)