Ferroli Modena 102 boiler pressure loss after switch off. New pressure valve/expan vessel prob unresolved?

Boiler goes off at 3.00pm and then need to re-pressure every half hour. About 6.00pm will re-pressure for last time as leave office then and no loss of pressure as boiler then comes on at 7.30am next morning but if not re-pressured will not come on. Can it be a leak if OK for 13.5 hours?