You should consider joining GLASS as well if you green lane:
www.glass-uk.org
We have many local reps who can advise on routes, pub meets, organised weekends and we're fighting at a government level to retain our rights to drive unsurfaced roads (it's got to be worth it just for that alone!).
Like GB MUD said, there's not a lot left in Sussex apart from a handful of byways and UCRs (although would have to check out my old maps from when I used to lane over that way regularly) and they're probably not suitable for you, well unless you get something you don't mind knocking about a bit!