Yes it is combi.
It definitely seems the pressure rises quicker once the boiler is off.
Checked the filling loop first, definitely not letting by, will suggest heat plate exchanger, thanks for your help.
HELP!! Had an engineer out who has done the following to try to fix this problem;
Recharged original expansion vessel to 1.0 bar which didn't work.
Replaced expansion vessel with new and charged at 1.0 bar, which didn't work.
Replaced pressure relief valve, which didn't work.
Checked pipes...