worcester greenstar 24i junior boiler stopped working?

Help I have a worcester 24i junior boiler 4 years old which the pressure keeps dropping had a leak a few weeks ago but this has been sorted, boiler started to rumble and bang early hours boxing day morning (26/12/2010), cant remember if pressure had dropped but I did switched it off, was advised by a retired plumber that this was due to a air blocked in pump/boiler who came and sorted it but just now (23:30 31/12/2010) pressure has dropped to zero no central heating (in 3 radiators) but still able to get hot water attempted to refill boiler up by water tap under boiler but boiler will not start up but will when hot tap switched on we get hot water (i've switched the boiler off at the moment), pressure was up a down for the last few days between 0.5 up to 1.5 as i've been keeping a eye on it due to being paranoid boiler will blow house up or hole on house. What could the problem be? and if needing parts what will/could this cost me