New passat owner with Heater issues please help

[Readie750]

Hello to everyone
I bought a tdi 130 sport estate 53 reg today, had a good look around before handing over the cash and its generally in good condition. When the guy pulled up in it was slightly disappointed as I wanted to check everything from cold but he told me he only lived 2 miles away and needed to put some fuel in it! How ever the air con was on and it was like ice so I was happy with that, but when I turned the heater up to (hi) the heater didn't seem to get even warm to be fair, the temperature gauge was sitting at 90. Some how I convinced myself it was likely to be the thermostat not opening properly and got some money knocked of the car and went ahead and bought it. When I got about 5 miles from the dealer the gauge went right up to the red so I quickly pumped the heater back up to HI and it was blowing hot air then once the gauge come back down to 90 so did the heater and went cold again. I'm hoping you guys can point me in the right direction and hopefully tell me it's the stat or an air lock or something quite simple!!!
Thanks