Firstly I am not a plumber, I know that we do not have any water tanks other than the central heating boiler. The water comes direct from the outside supply and is heated directly by the boiler.
The pressure (Even when boiler is turned off) has rocketed up to over 3 and the max is 4. the recommended pressure gauge is 0.7. I recently bled the radiators and whist I was bleeding them a large amount of water was released from the system. This in turn caused a drop in pressure. I opened up the water inlet valve (not sure if this is the correct term) to increase the pressure back to the normal level and all this seems to have triggered the rise in pressure I have now.