Thanks againHave a google of any open vented diagram, the F/E pipes are always behind the circulator on the circuit and never in front, that is the cause of your air ingress issues.
this is rather frustrating - seems like i have inherited someone's shoddy work ....
i have had a quick google and found there are examples of both ways though
the system does not do it if on central heating central setting - only on hot water - why is that ?
and does this suggest it might be possible to run this configuration and tune the problem out somehow - maybe by fitting a restrictor or something ...?
or is the only solution re-plumbing the F/E to alternate locations ?
thanks again for your help!