The layby that maxtorque mentioned is a good place to park and you’ll be able to get to the bike park easily from there. That will provide plenty of short mini dh/jumpy sort if runs which is enough to keep me occupied for most of the day!
However, if you’re looking for more of a XC/trail ride sort of thing then you can venture into the rest of the woods. There are many other slightly longer downhill trails that you can link up by riding along the sandy paths that criss cross the woods. The only problem is that some of these can be quite difficult to find and this is made worse by the fact that there are so many bridleways/footpaths all over the place.
My advice would be either to find some other riders to ask or follow the bike tracks (its very sandy so they’re easy enough to spot). Here’s a map to show you where you can find trails:
Trail Map
Best bet is just to ride everywhere and see what you can find really, the woods aren’t particularly big so you can’t really get too lost. Just have to watch out for horses and footpaths and those sorts of things because the it can get quite busy.