I've had two on the 1.8t...
Matt had one..
Some other chap had only just changed it and the replacement was faulty...
In this weather, when your sat with the heating on full bore, a faulty stat could easily stop the temp climbing over 70c even while stationary. He also doesnt say if hes tried getting the engine warm and then leaving it idling with the heating off to see if the temperature climbs to 90c...
Its a difficult call, and one could surmise that the car could probably benefit from both being changed anyway, but since its only just appeared in the cold weather, if i was asked to choose without seeing the car or running diagnostics, then i'd choose the stat.
If you want to narrow it down, get the engine hot (ie 10-15mins driving) then park and leave it at idle for 10mins with the heating off. If the guage displays 90 after that, then its almost certainly the stat.