Indeed, I hardly ever got a seat in the mornings from Hassocks. They may get the trains from Littlehampton but they are usually full before they get to Hassocks, Preston Park is a much better bet.
I used to commute from Hassocks when I was in Hurstpierpoint and the parking as OK as long as you get there fairly early (I used to get there at 7.30ish). It costs about £4.50 a day to use the station car park and that fills up pretty quickly, the best bet is to go on the southbound side. If...