OK, I have an Ariston Microgenus combi. It is dripping from the
pressure-relief pipe. The system pressure is pretty high and does not
go down despite the leak, which seems to suggest one of two things:

1) The filling valve it letting water through.

2) There is a leak in the secondary heat exchanger, so that mains-
pressure water in the DHW side is getting into the primary circuit and
and over pressurising it.

The first seems to be more probable, except that when I drain some
water off to depressurise, the speed which which the pressure goes
back up seems much faster than a slow leak from the filling valve
would cause.

Anyway, both parts are a bit of a bugger to get to and remove, so I
was trying to work out some clever trick to identify which it was
likely to be before I start unbolting things unnecessarily. Any ideas
from the panel?