Hockropper, theres quite a bit of good riding around worthing/brighton mostly on the South Downs, some nice trails around Stanmer Park in Brighton, also 30mile north is Peaslake/Leith Hill with a good selection of singletrack. Sussex MTB have regular evening and weekend rides, some based out of Quest (bike shop in Worthing), keep meaning to join in on one of their rides. BigSi posts on here so may be able to offer more on Sussex MTB.
The group I ride with have regular evening rides as well as local and North downs weekend rides, plus a bit of trail building/mnaintenance at Peaslake, your more than welcome to tag along if you want.