Geoff Clare wrote:
My boiler is working (it's heating the water that goes through it),
but something isn't right with it because quite often I can hear
the main gas valve turn off, and then on again about half a second
later, while the boiler is running.

Friend had a pretty ancient boiler (fixed pilot light job) that did the
same sort of thing. Turned out to be a dodgy connection to the gas valve
(either the external wire or the internal one to the actuator coil, I
forget now.) Quick fix and didn't do it again.

--
Scott