Cyclocross - CX and SSCX races and training

For anyone wanting to try cyclocross, or for someone who's done a couple of races and wants to get some more experience, then this Sunday's round of the London Cross League (www.londonxleague.co.uk) at Herne Hill includes a Go Race, start is at 1245.

Do we have to sign up beforehand or can we sign up on the day?

I need all the practice I can get before Muddy Hell.