Take the little air cap off the expansion vessel and press the little pin in, if there is water in there it is definately split. If you get nothing, then it may just need repressurising.
Is it working correctly because it is getting too hot and there is a problem with your thermostat, or is there a problem with your overheat stat because the temperatures are normal?
Firstly top your water pressure back up to 1.5 bar cold. Maybe the lamp is broken or the low pressure cut out switch is blocked on the back end? Let me know whether it works properly when the pressure is where it should be?
It definitely sounds like a circulation problem. Your rads are only hot at the top due to the lack of water flow. Your boiler keeps shutting down due to the fact it cannot get rid of the heat quick enough. Is your pump in your boiler or seperate?
Yes, it sounds like the body of the valve is stuck in one position. Remove the head, a quick spray of WD40 around the spindle and see if you can get it to move - gently at first, backwards and forwards. Also while it is off have a look at the head - turn the heating on and off and see if the...