Just bought a Octavia  1.9tdi estate mk2 . When I start the car the gauge moves slowly to 90⁰ then flush off the scale and throws the warning light up.

The coolant is fine it's not hot . This happens in a couple of min. 

The heaters don't throw out a great deal of heat either but I'm guessing the fans running don't help. 

I'm thinking Temp sensor switch and do the thermostat at the same time ??

Any other things it could be guys ? 

Nope, you have it there -

Thermostat.

And CTS. Buy a genuine CTS though, not a pattern copy as it will only last 6-12months. Maybe less...

Cheers mate . Just thermostat or replace both ? 

Well, id do both...

If its the cts you dont have a major problem it might just be the display circuit is failing and ECU sude is okay, thus no running issues. but if the stat is sticking you could have a major problem, as it may well be the coolant is overheating, but theres a bit of flow making it less obvious at the top end (resevoir)... so do both you have piece of mind that the reading is correct and it can flow.

If both of them dont change things, id say water pump isnt circulating properly.

I will replace both and top up the coolant . I drove 300 miles up the road like this and stopped at every 2nd service station to check the water level and it was fine . 

Today the gauge only moved to a 1/4 on my 30 mile commute. So I'm guessing it's best to replace both 

Well I got to the bottom of the strange temp gauge issue . Someone had fitted a cheap boost turbo chip. Took it out and the gauge stopped hitting 130° without the engine running. 

Fitted the new temp sensor but only hitting just under the quarter mark after a long long time lol. 

I'm guessing it's time to replace the thermostat now ? Looks a right pain to change ?