You've got an air lock in the coolant system - it's possible that the coolant has been drained and then refilled without being bled properly (they are tricky to do properly if you don't spend a bit of time on it, so it's quite common on the 400 / 45).
It's NOT the head gasket but get it sorted asap or it could overheat and damage the HG.
If you're doing it yourself, you'll need to drain the coolant, and refill. Catch the old coolant, biottle it and take it to your local tip (don't stick it down the drain).
Once drained make sure all the hoses are tight and the drain plug is replaced. Make sure the heater control is on the hot setting, and unscrew the bleed screw on the coolant pipe on the side of the block (passenger side).
Refill slowly until you see water coming from the bleed screw and the header tank is on max. Replace bleed screw and tighten. Start the car and give the top hose a good squeeze as it heats up. This should get rid of any remaining air.
Make sure you use the correct coolant - it's know as OAT and is red. Vauxhall dealers sell it if you can't get to an Xpart centre.
Whilst you're at it, order a gunuine MG/Rover header tank cap and replace that too. They're not expensive and should be replaced as you would a service item.
Hope this helps. I take no responsibility if you attempt to do this and it goes wrong!! I suggest you get a cheap Haynes manual too, if only to guide you through the process with pics.
Roverdose
October 2009
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