All,
I just wanted to thank everyone for all their help in getting to the bottom of this issue and am glad to say it has finally been sorted out.
The solution was to turn the pump down a notch and all has been well since. Having thought about it all further, my guess is that the problem may have materialised after we moved in because we have switched off most of the upstairs rads because we don't use the rooms. I wonder if this resulted in more pressure in the system and therefore why it started pulling in air through the vent. Anyway, switching it down from 3 to 2 seems to have done the trick.
Thanks again all
Lee.