Pressure dropping on worcester 241 RSF boiler?

Hi i hope someone can help me, the pressure has been dropping on our boiler and has noe gotten to the point where we cant use it, it used to drop slowly, but now once refilled from the loop, to 1.5bar in drops to 0 within around 30mins, and if the heating is turned on the pressure doesnt rise as it used to, although the radiators are still getting hot, the water from the system is also leaking out of the pressure relief valve outside, we have done some research and have charged the expansion vessel to 0.7bar as it was at 0 before, and there didnt appear to be any water when releasing air fom the valve, so im assuming the seal is all ok, the pressure is now dropping alot slower and water is leaking outside alot slower, do we need a new pressure relief valve? or could there be another problem? i am an electrician so have a basic knowledge but i am by far no heating engineer, any help would be greatly appreciated as i dont really want big repair bills, thanks jordan.