No inline thermostats on manuals, well there isn't one on my Ed30. Did you fit a genuine thermostat? Even the genuine thermostats are hit and miss, I had a faulty one direct from TPS and didn't have time to go through their warranty process, as car was booked in for mapping, so paid to have another TPS thermostat fitted and that ones better but not perfect. It occasionally dips a little bit and then recovers back to 90⁰. I'll have to book the car in to a VW dealer to have it replaced again under warranty. If you have VCDS you can see when the thermostat opens by monitoring the radiator outlet temp.