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Nick77
You say the plumber isolated the pressure vessel to rule that out?
Describe the vessel. It may be that the vessel is an accumulator that has lost its charge and therefore not delivering the necessary flow
Hi Kay-Jay and thank you for your comments.
I was hoping that the pressure vessel (5 gallons) was the root cause of this as it would have been an easy fix. It is under pressure and I tested the schraeder valve; there was a slight release of air as expected but no water came out so the diaphragm is probably intact. The second plumber temporarily disconnected it from the circuit - there was no discernible difference to the flow. In the last couple of days the flow rates have deteriorated and there is no hot output from the bathroom mixer tap. It's all a bit confusing. I'm now hoping it turns out to be an air-lock as it would be the next most straightforward problem to deal with though I admit that may be due to my overall naivete in these matters.
Thanks again.