Ravenheat, The Silver Star LS 100T keeps blowing main breaker?

The fuse kept blowing some time ago but eventually settled down and has not been happening, it hasn't happened for the past four months but now has happened twice within a week? I believe the breaker has a 15 amp fuse wire but I have looked on the web and cannot determine if this is correct so I guess the main thing is to be sure the correct wire is being used, further to this the pressure on the boiler has always dropped gradually to the point where I have had to increase pressure every month or so, the plumber who fitted it a couple of years ago actually left the flexi pipe connected and showed me how to increase the pressure but I have been told that he shouldn't have done that?, I have searched all radiators and visible pipes without any sign of a leak and there are no obvious signs on the ceiling etc but having to repressurise hasn't really been a huge problem, the fuse constantly blowing is becoming one, the last time it happened the boiler was completely idle and so I don't understand how the fuse could blow?
Your help/advice is greatly appreciated, thanks