Hi Arch
Last year the barriers and pinch points were removed from the path, which I think was a good thing (they make passing with panniers tricky, and I am sure more so with a trike or recumbant). I am not sure they would have really stopped motorbikes anyway, particularly mini-ones, and there are also many "unofficial" access points through the trees anyway.
Last year the police were out on foot and with a van attempting to catch 'em on a couple of occasions (not sure how successful they were), and they put warning notices up, which were vandalised.
I don't think it is happening too often, but when it does, and a motorbike zooms past totally without warning, it is quite scary, more than if one passed on the road as they get very close, the path is often narrow, and they tend to drive crazily.
I haven't yet had a response from my email to the policeman (maybe he's on holiday) so I will try and escalate it with them.