If you have a leak somewhere that's letting air in, it could be anywhere
in the system not necessarily in the boiler

It's not a pressurised system either, so could be anywhere :-}

You might find that your auto bleed valve isn't working properly, it
will be somewhere at the top of the boiler and is usually gold in colour
and circular with a screw top an inch and a half in diameter and two
inches tall

I'll check it out over the next few days...

Check out the joints in the boiler to see if there is evidence of water
leaks especially around the heat exchanger
The only other (continually) moving parts in the boiler are the fan,
and the pump if its part of the boiler

No sign of leakage internally, and the pump is seperate :-}