air lock matey!! you need to top it up via the coolent pipe that goes from header tank 2 the turbo,
first drain all coolent bk out, then do the following....
remove coolent hose from header tank side, then remove the top little radiator hose on top drivers side of rad (goes bk to header), then either use a funnel or the hose pipe (if the engine is hot use warm water) put funnel in the turbo hose, start to top it up (dont let the funnel run dry) keep topping up till it starts to trickle out of the top of the rad, put the little hose bk on and keep topping up till the header is at the right level, refit the header to turbo hose,
get in the car and turn heaters on full warm. fire car up and leave it running for about 5-10mins , leave the header cap of the header. once the car is up to temp and blowing warm heat through the blowers replace the header tank cap. take her for a run keeping a look on the temp,, all should be well there after.
hope this gives u an idea