got to the bottom of this problem, thought somebody may find this useful in future maybe? basically the airvent was letting the air back in (presume this is not supposed to happen so this may be faulty). closed the airvent body completely but keep the red cap on top open half a turn. now there is no air coming out of the taps, radiator(s) don't need repeated bleeding and pump is quiet when it fires up on a morning. cheers to everybody who gave advice.
