Your expansion vessel may need looking at. It could be th emembrane has failed or that it may just need properly repressurising with air.
Search fopr testing gas boiler expansion vessel.
Your own boiler may be a little different but there is probably an expansion vessel inside the casing with a Schrader (bicycle style) valve. If you take off the dust cap and use a pen or small screwdriver to press the centre and release a tiny bit of pressure, only air should come out. If water comes out, the membrane inside has failed and the vessel will need replacing.
If OK, you MAY be able to repressurise with a bike pump. Yyou may have to drain a pint of water or so first, pump the vessel then prepressurise the system. Read the installation instructions. I fin doubt ask a central heating engineer.
Dunno how expensive a new vessel is fo ryour boiler or how much of a pain it would be to replace. Some people just fit a new external one
ddfbh, December 2017
