the pressure rises when the heating is off. I think that maybe the pipe you use to refill the system is leaking and adding pressure to the system making the Expansion Vessel expand and leak slightly due to high pressure. When the Heating is on the pressure goes back to normal .
Is it possible to unscrew it and change it myself?

Martyn, November 2011

Yes, BUT

Why the expansion vessle???
If the refill pipe is leaking. 1st officially it should not be permanently connected.
BUT Heating on, pressure WOULD increase, due to the temperature of the water. increase temperature gives increased pressure.
Heating off, it should drop.
Pressure should never go to a max of 3.5 bar, as that is when the over pressure valve would open. You should have a cold pressure of about 1, Turn your heating etc off, at the control, not the boiler, you need to see pressure. WAIT until system is cold, not warm, cold. If pressure over 1, open a radiator bleed valve, catch the water that comes out. get it to 1 bar. try that, if better, you may need to drain the system completely, and refil. IF you have filled the Expansion vessle with water, it cannot do its job, (which is allowing by the air inside, to absorb the increase pressures of the system. Expansion, says it all, allowes the water in the system to expand, which keeps the pressure from going too high) Water cannot be compressed, So if no where for it to go when it expands, with heat, bang !!

SD, November 2011