Presure reducing after use on Worcester Bosch 28i junior combi boiler?

The pressure guage on the boiler keeps going down which would suggest a leak. However when the system is checked a leak cannot be found and all the radiators are full.
When water is introduced into the system it retains the pressure when the heating is not on and only after the heating has been on and gone off starts to lose pressure.
The guage goes down to just under 1.5 bars and we get irregular hot water. When the heating comes on the pressure can rise to 2.5 - 3.0 bar.