Stanmer park is king for singletrack, and you can park at the sussex uni for free at the weekends (as long as there isn’t a football match)It’s not too hidden but your best bet would be to find and follow someone to make the most of it.
The Wildpark next to stammer park is your best bet for short DH runs, plenty of berms and small jumps concentrated into a few min run should put a smile on your face.
In the same ride you can also easily hit the south downs, very open and long shallow climbs to clock up some miles.