Aug 1.8t cooling system problem help please

orite people first post and sadly it's about a problem and not sure if it's in the right thread or if there's a post up already about this but any help is greatly appreciated.
Thr problem is the header tank keeps throwing water out of the over flow after the cars been run for a while then parked up. It only overflows when the cars not running like an overheat but the temp gauge only ever reads a max of 90 and I've replaced sensor and thermostat and the header tank any ideas as to what could be causing this??
Thanks
Jonny

Yeah I did and I was slightly annoyed that you have to buy them separate

Hi thanks for the help. I've managed to resolve the problem. There was an airlock in the block that was stopping the water reaching the thermostat. So it wasn't opening and letting the water cool. I fiend the problem by checking the water pipes and the rad feed was cold so replaced thermostat but still had the problem when I checked other pipes the return pipe from the oil cooler was also cold. I pumped it a few times bubbles came out the header tank and everything stated flowing again. Also I had a problem with the fan nor kicking in and the temp gauge not rising above 90 even though it was hotter. The problem was the parts store had given me the blue square 4 pin sensor when it needs to be the green one replaced this and everything is running as it should on the cooling system. I now have another problem which I am lead to believe it's quite a common problem but I never see any conclusive solutions. When I lift off the throttle the car jerks quite badly but not all the time. I was told it's the turbo not dumping the boost effectively but I have checked all the pipes and replaced the dump valve. I can cope with it but would prefer if I could fix it. It's stage 2 remapped if that could have anything to do with it. It was mapped for the k04 but it didn't like it so had to go back to the k03s. Thanks in advance for any help appreciate it
Jonny

Also has anyone done a centre lock wheel conversion? Was it easy enough and roughly how much would I be looking at?
Thanks
Jonny