Ferroli Optima Combi losing pressure

Hi, I have a 7 year old Ferroli that started to gradually lose pressure earlier in the year, but not so much that i was concerned. After switching back on after summer it's now got to the point that when it has cooled down after the heating has been on it's generally lost between 0.5 and 0.75 bar on the pressure gauge and hence I have to re-pressurise every day. I've checked under the floor and have no ceiling damp spots so I'm pretty sure there isn't a leak. Any suggestions? Thanks in advance