Air in system?

Hi all,
I've got a glow worm fuelsaver 45f that I think may air trapped in it.
Its a pretty standard open system
A couple of weeks ago the pump gave up,this was changed and ever since,the boiler makes a bit of a gargling sound when its fired up and the overheat trips out.
Is it possible their is air trapped in the system,and if so,how do I vent it,
Cheers
Gary