Tom,
Like Liz says, if you're just going for the weekend then you can just turn up and pay to park and buy a ticket on the door (300FF).
You might have difficulty finding a ferry at this late stage but the tunnel should still be good.
Other than that it's easy, just follow all the other British cars from the port...
You'll have to sleep in your car or sneak into a campsite on foot (although by the day of the race there won't be any space).
Check out the Circuit Guide on this site, as well as
http://www.club-arnage.com for all the info.