Pressure in the combi boiler keeps falling rapidly around 1bar/day?

Dear All,
I have a worcester combi boiler 24siII model, over the past 5 weeks , I am noticing the pressure keeps falling down around 1 bar in 24 hours. I am normally setting the pressure around 1.5 to 2 and end up refilling the next day. The pressure gauge has a bold mark between 1 and 2.5 marks and I assume that a normal setting should be round this. Can somebody please advise me whether I need to take any further action. Thanks