i can asure it nothing to do with water pressure,its were the check valve shut off and stop any more water going though.its excess water i bet its not even thimble amount.its wear on the check vavle,as i said nothin to worry about.
maybe to late but anyway,i recommend you call a competent engineer.but just to point you in right derction we add problems with aps (air pressure switch)you can check this by pressing+/i which give you d code press + untill you get to d24.press i then run a tap.you should then see 1.which say...
maybe to late but anyway,its a poor made boiler,glowwom add many issues with it.cant say wot may get myself in trouble.as glowworn are owned by vaillant i would recommend a vaillant ecotec 831.combi.may cost abit more but well worth it.
this may help ex vaillant/glowworn engineer.
you may have it sorted but anyway.as ex group service engineer(vaillant+glowworm)thought i may help we add many problems with check valves.you can get a filling kit.comes with new tap +new tube and check valve.