seen a few threads on here discussing
how to diagnose "thermostat" problems. The old way of doing it i.e. pre vagcom etc. is to start your cold car , and hold onto the radiator hoses. If they gradually start warming up straight away then your thermostat has failed open and needs changing,
They should stay cold until the thermostsat opens at which point they change from cold to pretty hot in a matter of seconds.
Just a thought for those in doubt what part to change first.
p.s. my 2000 reg 1.8t cupra is on about 90000 miles and I think the stat has failed open . ie. takes ages to get warm and doesnt stay at 90. I will now try my own diagnosis!!