Thermostat

[haydn callow]

If you get a old innertube off a push bike, cut out the valve ,(the long thin racey ones) they screw in nicely to the headertank overflow (just under the pressure cap)
Do this then just pump it up till the cap leaks. That should show up any leaks in the system and get it up to full pressure.

[Nobbybongo]

Thanks will try it out, much cheaper option than buying one.