Thanks Gas Guru, that makes sense... spotted there's a cylinder in another cupboard nearby! Seems to be an open vented system then. No error codes / flashing light so nothing for it; have called an gas boiler chap out to take a look.
Thanks again anyway
My tenant has a Worcester Greenstar 15ri pressure gauge that isn't heating well (was serviced not long ago) and the symptoms sounded a little like low pressure.
However, there's no pressure gauge on the unit itself nor an obvious water pressure valve / key.
I found some online manuals...