Hi the pressure gauge will increase slightly by .25bar if the hot water tap is turned on during a heating cycle because your boilers diverter valve closes off the flow port to the heating circuit and concentrates sending this heating water through the boiler main primary exchanger and hot water platent exchanger ,to produce instantaneous hot water .This change of water circulation provides less resistance to the pump which is capable of 5 to 6 meter pump lift at low static head and low flowrates therefore because of the small amount of water volume reqd to circulate through these two exchangers the pump pressure characteristics naturally increases the internal pressure in the boilers waterways mainly because the pump speed do not reduce accordingly when this changeover takes place .
With regards to your pressure loss check the overflow pipe isn't leaking on the outside of the property this will be a 15mm copper pipe usually a meter or so below the flue terminal ,and see what pressure the gauge goes up to when the heating gets to 82deg c if it rises to 3bar and above I would definetly be looking at recharging the expansion vessel , let me know if I can be of anymore help , Boiler Medics