OK,
so this may well fall into the basket of really obvious but, it seems that Dynamic Registration Hates no-cache setting.
Once I set it back to light every thing springs back to life.
It does cause me a couple of issues relating to profile properties not appearing to update in real time but it sure takes the sting out of the issue.
I had noticed such a significant performance hit in previous versions though and it would be good to be able to switch back some time soon.
Regards
James