From Upwaltham you will pass Bignor, which has a Roman villa. Stane Street, which is a 56 mile Roman road that linked Chichester to London.
After this there is a road crossing at Bury A29, take care as it’s a fast road. Then there is a nice downhill sections, mind out for the ruts, leading down to and crossing the River Arun and Amberley village. At Amberley there is a mainline train station, cafe, pub and the Amberley Museum, which is great for a visit. They have working attractions, and also they filmed the mine and airship section of James Bond, A View to A Kill there.
Water tap near the sewage treatment plant! After this cross the road and proceed up the very steep Amberley Mount, may be a walk rather than ride. Then follow the sweeping SDW all the way along the ridge, some great views down to the coast of Bognor, Littlehampton and inland to Storrington.
There is a car park at Kithurst Hill, with a road leading to Storrington.
If you have time there is a World War II Churchill Tank just off the SDW, well worth a visit.
The South Downs were used for training during WWII, the tank broke down, and was used for target practice (lots of bullet holes). After the war they were unable to move the tank due to the terrain, so they rolled the tank into a bomb crater. Until 1993 when it was unearthed and left where it sits today. Parts were removed to restore other tanks.
Water Tap: Amberley near sewage works (and Café)