Thanks for the replies )

I did bleed a reasonable amount of air from one of the rads yesterday
(it seems to acculumate in just one rad due to the system layout), and
noticed the pressure may have dropped very slightly, so it could be a
small leak somewhere on the pipework that is not visible.

I just surprised that air can still sneak in on a pressurised system,
and that it can make quite a noticeable amount of noise. I'm guessing
this would be a very small, slow leak due to the lack of immediately
noticable pressure drop. The problem did go away about 6 months ago
(after the system was pressurised), but seems to have come back after a
couple of rads were added to the system (all the new pipework looks
watertight).

I'll give the leak sealer a bash and see if that resolves, I assume
this stuff is quite effective and has solved similar issues in the
past?

Many Thanks,

MH