Glowworm Compact 100e?

Went to a Glowworm Compact 100e the other day and found out it was a lose electrical connection. Problem solved i thought, however was called back the following day as the boiler failed to light on C/H mode, found out that the pressure had dropped, repressurised the system and waited to confirm that the heating came on and yes it did, however i did notice that the pressure kept on rising while on heating mode, then the PRV cut in and the boiler lost pressure as it should. Dont understand why the pressure keeps on rising when the CH is on and it drops when its off. The Room stat and boiler stat are working i think it most be something to do about the expansion vessel.

Anyone shed some light on why a boiler pressure keep on rising when on CH and dropping when CH is off.