B&Q Silverstar Boiler, main fuse blowing, losing pressure slowly?

Hi - All help is greatly appreciated as I am lost at the minute with this problem.

My Boiler stopped working and we eventually traced the problem back to the main fuse box, the 15 amp fuse keeps blowing.

Is 15 amp even correct as somebody suggested it was too high but surely if I use a lower amp wire it will break more often won't it?

The heating runs OK and the fuse doesn't often blow when just that is running, the hot water from the tap in the kitchen also seems ok but the shower which uses hot water from the boiler has blown it a couple of times, usually after being on for 2 mins but it has seemed fine the last couple of times.

Basically the fuse seems to blow either when the shower or hot water is turned on and reaches full heat or strangely when the heating system is turned off after being on.

From the 15 amp fuse wire in the main box the wiring then runs through a 13 amp fuse spur which used to run the old immersion and then to the boiler itself which I beleive is protected by two fuses within.

I believe there is a problem with the pressure as it seems to be dropping over a few days but I cannot find a leak and would rather not pull all of the floor boards in the house up unless absolutely necessary, the plumbers who fit the boiler about 2 years ago showed me how to increase the pressure in it using the two valves attached to the flexible pipe which I beleive is joined to the hot and cold pipe inlets (I think).
A plumber I know said this pipe should not have been left connected as there is a very small chance drinking water could become contaminated, when I do add pressure the top valve drips quite a lot of water and because its only a half turn from dripping to not I wonder if pressure could be lost here?

When the boiler is off the it often reads around 0.7 bar but when turned on it rises to what I guess is the actual pressure, the thing is I'm not so sure the reading used to change so much when it was off or on before these problems, when on it can gradually rise to between 1.5 and 3.5 bar and seems to runs fine between these, I used to have it around 1.5 and 2.5 but when I add pressure now it doesn't seem to give a very acurate reading straight away, should it be off or on when adding pressure, does that matter?

Finally I did notice when the hot tap was running in the kitchen where the boiler is, I could see the pressure dropping very slowly, I've been told that a boiler may need a pressure increase from 6 months to a year but not Daily or weekly.
The difficult thing is I don't know if I need a plumber or an electrician and I can barely afford either right now, If I need to then I'll do that but I'm hoping you guys can give some advice and point me in the right direction.

I have replaced the fuse wire about 5 times now and I know this cannot be good, It's just so cold at the minute and I gotta shower for work! all of your help is greatly appreciated - Thank you