Yup, I read all the other threads on this topic and am no further forward.

Just a wee update on this one. Since posting that the new cap hadn't helped, the cap wasn't opened until yesterday, after a fair amount of driving. Normally there would have been quite a large rush of air when opening but there wasn't, just a very smal breath. This would suggest it reached a level where the valve kicked in at some point between Thursday and now.