In message , RB
writes

Close to the pump, I have what I think is an air separator. It's a
cylindrical object (about the size of one of those enamelled tin mugs)
with 4 ports. The pump seems to circulate water between 2 ports on one
of the flat ends. A 3rd port at the opposite flat and 4th port coming
off the top but off-centre both go up into the loft to the header
tank, one to the header supply, the other to an open vent. If this
thing is an air separator, then it's not working as you can also hear
the air whooshing through the circuit.

Can someone confirm it is an air separator and do they fail?

Yep its an air separator. There is nothing _to_ fail. Its just an empty
pot. The water swirls round the outside (of the inside, IYSWIM), the air
bubbles migrate to the centre and disappear off up the vent pipe.
It is possible though that the vent or feed is blocked or restricted in
some way.
--
steve