It sounds like your pressure vessel is ruptured. A common problem with combi sealed system heating systems.
I have experienced it many times.
When you said you have to keep topping it up and then when you said it was high.
Whats happening is... when the heating comes on and after you have topped it up, normally as it heats up the pressure vessel takes the expanding water. You cannot compress a liquid but you can a gas.
The pressure vessel has a bladder in it which is full of air. Its sealed and connected tot he heating. When the heating comes n the expanding water needs somewhere to go, it compresses the bladder in the vessel. The pressure in the system actually stays about the same. When the rads cool down the water retracts from the vessel as the bladder re expands.
When the bladder is ruptured the water fills the vessel, but can''t expand any more, there is always a pressure release which expels water outside. After you have filled the system, the water expands but with no where to go the pressure relief valve opens letting out the pressure, when the system then cools there is no pressure left as the water was expelled. This is why you have to keep filling up, but when your heating gets hot the gauge will show high, this will be when the valve is opening letting the water out you keep putting in.
I hope you get that, it is basic physics. You can get pressure vessels from ebay and you do not need to get the same one for your boiler, many plumbers fit any vessel and fit it exterior to the boiler, this is fine. All it it is a vessel with a bladder in you pipe up to the boiler or anywhere on the rad circuit. You can even leave the damaged one it, it will be fine. This is your best repair and can be done for less than £100. If you wanted to get the actual vessel for the boiler it could cost hundreds. I did mine myself and it cost about £70, although I did get the specific vessel off ebay.
Thanks Bobbinalong - very helpful. It does sound as though that is the problem, and if I understand you correctly I don''t need to do anything about it. If it is not dangerous or affecting the functioning of the boiler, then I would rather not pay £100 for a boiler on a house I am going to have to leave shortly. If on the other hand it is the difference between heating and no heating (esp as i have a lodger) then I guess it would be money well spent for the coming cold months when things will be miserable enough without freezing!
Thanks for taking the trouble to write and explain.
you don''t have to do anything about it, its just a pain having to fill the boiler every time.
Some boiler don''t have a cut out on low pressure, some do. The best way round this is to leave the heating on all the time on low, turn the room stat up a little if you need to if there is one, or if not turn the boiler stat low. Its actually not that much more costly to leave the heating on all the time on low, especially if someone is around generally. If the insulation is good, the house will not lose too much heat, keeping the heating on all the time on low, means your not heating the house from cold every time.
Your heating is losing the water as it heats and cools then heats again, if you left the boiler on low, you would not need to keep topping up.