I had this happen to me, the cap seal didnt really 'stick' to the rocker cover so i put some thin fabric on to hold the cap into place, while i was waiting for the AS st/st cap to arrive as that has a grub screw to keep the cap in place.

http://www.autospecialists.co.uk/CVH...oduct-233.html

A good mod is to fit the AS breather and kit, its better than the baileys one and includes the outlet for the inlet manifold that the baileys one does have.