are you actually using an atmospheric dump valve or is it a recirculating/diverter valve?
If it's a dump valve then that's most likely the problem, swap back to your plastic DV then order an uprated DV such as a Forge 007p.
If the bailey valve you are using is a DV and is working OK (just whip it off and check the piston isn't sticking and that the spring feels reasonably firm), then there are lots of things to check; really you'll need to get it on VAG-COM to check for fault codes.
I endeded up resetting the ecu by dissconnecting the battery this cleared the problem but will get the dump valve off as soon as possible and fit a forge 007 DV instead as that sounds like this is causing the problem.
Many thanks to all who helped me with this very grateful.