Following two recces I have shortened this to about 6.5 miles. Please book by text before Monday 6pm as I will not do this walk with no bookings!
Buy a Thameslink return to Brighton (£13.90 with SRC) and a single from Falmer to Brighton (£2.50 with SRC). From Harrington Road Bus Stop take 272 bus at 11:23 to Rockrose, next stop after Pyecombe garage.
Long gradual climb to Ditching Beacon (possible ice cream stop) then down to Falmer skirting University of Sussex. Possibly pub stop for drinks before walking to station where frequent trains to Brighton (£2.50 with SRC) then Thameslink from Brighton about every 15 mins to East Croydon and London. Bring picnic.