Thanks again.
I'll do it with numbers to stop me rambling
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1) The water doesn't disappear overnight - it shoots out the overflow pipe pretty soon after the boiler reaches top temperature.
2) I start it at 0.5 bar, and it runs perfectly at 1.5 - 2.0 bar, never above - so if the pressure relief valve is venting it is doing it at too low a pressure. I have ordered a new one given they are not much more than a fiver.
3) The old permanent filling loop does have all the right valves in it including a pressure reducer marked 3 bar - but when i turn it full on, the clock pressure does not rise, so its not doing its job for some reason. Maybe it cannot work cos its on the flow side of the CH ? This is a bit of a red herring anyway as i'm aiming to make it redundant.
4) Today's news : i managed to get the plate off the top of the boiler which covers the chamber at the back where the expansion vessel is housed - took a bit of demolition as the previous occupants had dry-lined over the back two screws ! This revealed that yes, the expansion vessel is kaput - i pressed the schroder valve in and H2O came p-ssing out immediately, so i'm presuming the diaphragm split aeons ago and the whole EV is full of water.
5) So the pressing practical question is one you referred to - how about fitting a new EV near the boiler on the return side of the CH. Now that i've found the existing EV is totally full, i'm not worried anymore abut it still playing a part in the system - Except...IS there any connection between that full EV and the constant loss of water, or is that just a faulty PRV ??? Does the fact that there is nowhere in the EV for expanding water to go force it out of the PRV into the outside world, even though the clock pressure never shows more than 2.0 bar ? (so shouldn't the prv stay shut?) Having never worked on one before, i've no real idea about the pipework running from the expansion vessel and how it empties to the outlet pipe when it gets full up.
I will return - gotta eat!
Cheers.
ps: just noticed yr "an adjustment knob integral to the [pressure reducing] valve.". Thanks - something else to find and play with.