Over-pressure on Worcester Bosch Heatslave 15/19?

I'll give you the full story. 2001 Worcester Bosch Heatslave 15/19 oil combi boiler; the pressure gauge showed repeated pressure loss over a period of a week, from 1 bar down to 0. I topped up the system each day back up to 1 bar. No obvious internal or external leak is apparent. Then we ran a bath and had no hot water, and radiators were extremely hot. I checked the pressure gauge and it was at 3-bar with the pressure-relief-valve venting. We switched the boiler off, the PRV continued to drip until the pressure reached 0. I topped the system back upto 1-bar, switched the boiler back on, the pressure rose back up to 2.5 bar and held reasonably steady but once switched off the pressure dropped to 0. I had similar symptoms in Nov-09, we cured the problem then by replacing the expansion vessel. I have now fitted another new expansion vessel but this time the fault remains. I've noticed the PRV will often allow water to be drip released even when the pressure is below 3-bar. And when hot water is taken from a tap the pressure gauge will drop by about 0.25bar. I'm thinking the cause is either the PRV or heat exchanger. Any advice would be great.