Park at Coldharbour by the pub. Take the road opposite the pub after a couple of hundred yards it becomes a bridleway follow it and keep (but do not turn)left shortly you will see a signpost for Summer Lightning on your left.
When you get to the very end of SL turn left again and follow the steep hill down to the T junction (all bridleway) turn left, ride that up to the tower and then have a chat with whoever about where next. I suggest heading down the hill too the farm and then down the mother into Holmbury etc. Leith, Holmbury, Pitch is a nice few hours ride. Go to Nirvana Cycles in Westcott and get a map.