How can I fix hot water problems in my glowworm75pcompact condenserboiler?

I have a 5yrs old glow-worm 75p GC# 4704703A condenser boiler that has developed problems sustaining hotwater output since this winter. We presently have to turn off the heating which works very well to get good hotwater if at all. I also noticed the pressure indicator has been dropping slowly requiring refilling more often than necessary. Any advise would be much appreciated.