As everyone else has been saying, You’re not allowed to ride on the ramparts of Cissbury ring, one of the reasons was that a few ( well, possibly 10 or 12) years back some people had effectivly cut a pump track around the whole ring. I didn’t know that the ban on riding through the centre has been lifted though as I thought this had been downgraded to a footpath, but to be honest I can’t really see the point of doing that as there are better and quieter trails in the surrounding area…
Oddly enough, the Strava segments shown on the google earth pic are in that pic don’t show the two best (and legal) ones.
Still, all the trails up there are a bit sloppy and slippery at the moment.
Oh and the SDNP authority has a land access rep who does trail running and mountain biking. When the authority was being set up they were (and still are) extremely supportive of the mountain bikers, just remember the forum is not the Park Authority, just interested parties.