Fear the worst!!!

The car has been abit weird lately, start it up and drive it and water temp goes half way as normal. Put heating on and cold, until about 2 mins after water temp has gine half way then its hot. If i stop in traffic the heating goes cold again. Water temp will shoot up just under the H mark on gauge til the heating gets hot then drops back down to halfway. It keeps doing this til engine knocks off. I bought a new thermostat yesterday, fitted it today and now when ive filled water back up, started the car, got air out there seems to be no water getting into radiator as its cold even when temp goes half way. I thort it was air in the system so did it again. Same thing has happened. I took it for a drive and temp went just under the H and the hearing was cold, it wouldn even go warm.
Im thinking its the headgasket now but no oil in water or vice versa, im totally pisssed of as need car for work etc. im going to let it cool down and check water again soon. What else could it be if not headgasket.

We had a nightmare getting the air out the banger when we put new rad and low temp stat in. It just went all of a sudden

Sounds like the water pump has failed.

Been to see Mr sawkins over the road, he came out, started car up every pipe heated up apart from the return from Rad to expansion tank. Its blocked up with summit so gonna wait til car has cooled and have a prod about. Still abit of air left in off me but thats sorted. Just that pipe from rad to expansion tank is blocked or there is summit at the top of rad.
Thanks for your help guys as i was panic stricken lol.