The Racecourse P&R works well, or you could try Bexhill Road in Woodingdean and then walk down the public footpath. It's a bit of a slog uphill after the game.
The Seaford train normally waits about 5 mins at Lewes to connect up with the London/Eastbourne train arriving. The fact that this train didn't give enough time for everybody to be able to get off, is entirely Southern Fail's fault.