Spotted...

Sadly, not. VCL organise a couple at the end of August, early September to get people in the mood.

There are fairly regular races at HH as part of the London Cross League though:

http://www.londonxleague.co.uk/

And Muddy Hell III (on October 22nd) has a go-race for those new to cross.

Can you have a go on the cross section out the back of Herne Hill on your own, or does there need to be some element of supervision due to it being a cycling facility?

I'd love to go and thrash round the course for a while just to get familiar with the bike.