Pete
Sound like your stat is malfunctioning.
If you see bubbles in your expansion tank this means that water is being forced the wrong way around the cooling system. This is caused by a thermostat jamming shut and water is unable to move around the system in the correct way. Steam bubbles form as the coolant gets too hot and they rise up thru the easiest path to the expansion tank.
Usually stats fail safe i.e. in the open position which means that the engine will always be over cooled. But some can fail closed.
Open her up and horse out the stat. Check it by suspending it in a pan of water which you bring to the boil. The stat should open at somewhere over 90 deg C (I don't know the exact spec for your car) use a cooking thermometer like one for making toffee to check the opening temperature.
Mind you if you have had the stat out to check it you might as well put in a new one.
Also disconnect one of the rad hoses and stuff a garden hose into the radiator outlets and back flush the system for half an hour to get rid of any sludge in your block.
Make sure you refill her with the correct ratio of antifreeze/water.
Andrew