It could just be air already in the system, finding the high spots to collect. It can take months to completely bleed all the air out of a system.
It's been 2 years since he new boiler now, so I'm kind of assuming that's not it?
Hhmmm, spoke too soon - it's still happening. There was always a random element to when it would happen.
I'll try turning the pump to its lowest setting... plumber says pop into the attic and see if water is dripping into the header tank, which of course should tell me if there's a leak somewhere