300Tdi overheating

Good evening, just looking for a bit of advice on my Disco 300tdi. It started overheating the other day with the gauge going into the red when I first noticed it. The car was running perfect. There is no loss of coolant but there is small air bubbles in the expansion tank when the engine is running.
The other thing is the heater is stone cold even on the hot setting.
Suspecting a blown head gasket I opted for the "Steel seal" treatment which a lot of folk have sworn by. However I have just taken it for a run and exactly the same symptoms. The bit that puzzles me is that the car is running perfect, no loss of performance, certainly not that I notice.
Any suggestions would be really appreciated and the car is not worth sticking into a garage for a big repair bill...
Thanks in advance.

Check the engine bay side of the radiator. Do it properly, take the shrouds and fan off and have a good look at the fins .. When mine overheated the front of the rad was fine, the rear had virtually no fins at all left! The rad also includes the oil cooler, so it needs to be in good condition.

Head gasket repair in a bottle may cope with a small failure never had any luck with it myself.heaters not working as exhaust gas getting into cooling system and will collect there.you could try a sniffer test kit.

I would say head gasket all day long, main reason on the 300 is P gasket weeps people dont notice, coolant gets low, water pump mounted high so no water at the pump = no coolant flow= overheated engine = knackered gasket, this might have happened before you owned the car and has chosen now to show the symptons.
Gasket is easy to do

when youve replaced head gasket ,check rad gets hot all over if it doesnt replace it,rads have caused most of the failures i used to see ,now its just age a gasket will only take so many heat cycles

Hi guys, thanks for the detailed replies and got a better understanding of the symptoms now. I am planning trying the gasket replacement myself...

Check water pump i had it on my old tdi thought it was head gasket turned out to be water pump

This could be a lot of things but worth checking the heater hoses. Mine leaked where it joined the head.
Could not find it and it led to a cracked head. I've thrown that pesky engine mat thing away now!!!!!!!