Boiler on the blink, help needed :)

[slappy]

I am not a heating engineer but I had similiar problems with a Baxi a few years ago, pressure would drop over a few weeks. To solve the problem I had a new pressure vessel fitted, I would not have done the job myself as the engineer had to remove quite a few parts to get at it.
If it isn`t that it might be a leak as others have said, finding the leak is the fun part.