Thanks Gav. The pumped pressure is set to sit at 3bar. Normally the
pump cycles on again at 1bar. Over time to pressure drops from 3 to
1bar more and more quickly. After running down the pressure, by
opening a tap with pump shut off, applying the tyre pump to
re-pressurise the 'empty' vessel from 1bar back up to 3bar the fall in
pressure is slower for a similar rate of running a tap.

As for leaks I have no evidence of any; and each night, & on weekends
away, the whole system is switched off (we are slightly paranoid) with
no pressure loss at all.

There is no makers name on the pressure vessel nor on the plumbers
invoice. Is there any diagram available of what is going on inside
these things?

Thanks

Phil