Many thanks for that. Yes, a faulty expansion vessel was at the back of my mind when i posted the query. I've only just inherited the system via a house move, so just listing the faults with the 105e Baxi, which although probably ten years old does still do its job but in a messy way! Leaks from too many junctions etc, and leaking out of the hot water diaphragm housing pin hole which i know is a common problem. Pity the expansion unit is a boiler-off the frame job. Haven't added any extra rads and i don't think there are too many on the system. The boiler is maintaining pressure even though the prv lets by at hotter temps, because a previous fixer has placed a permanent flexible filling-loop bewteen the cold water supply and the flow (out) side of the central heating circuit - which is as far as i know on the wrong side ! (And shouldn't be left connected anyway). So it's working in its own eccentric way. If i could ask a supplementary - one of the list of jobs is to fix up that leaking hot water diverter valve diaphragm, and i've acquired a new one. According to bits i've read, including the Baxi service manual, to get the unit dry before opening up, one just shuts the cold water-in isolation valve, and opens any hot water tap in the house until it stops running - no need to drain down the boiler. But when i tried that, the hot water wouldn't run at all - only running from the kitchen hot water tap when i turned the isolation cold water-in back On. I was stumped by this, and my only guess at a reason is a massively furred-up automatic air-vent right in front of me which is another job on the list. If that is totally blocked could that be stopping the flow from the hot water side of the boiler to the hot taps ?