Not my area but I know some of it, the river Ouse through Lewes and the Arun running through Arundel. Some of it is free fishing but its tidal which makes things a bit difficult but, roach, bream, mullet in the summer, sea trout and sea bass run through as well. I have fished in Lewes on the tidal stretch a few times and the Arun at Arundel last summer, did not do a lot of good but the local tackle shop in Lewes said good roach and bream catches can be had. There is also the Cuckmere which is mainly club controlled, this runs out at Birling Gap, there are some tidal stretches of that as well that might be free. I would google these for fishing and maps which wil give you more information about clubs, free stretches and so on. Away from the tidal stretches these rivers are mainly run by clubs but might be worth joining one if your going to do a lot of fishing. Barcoombe I think it's called above Lewes might be worth looking at.
Another might be the Western Sussex Rother, location, clubs etc
