The meeting point is the Rose & Crown Pub, Farnborough Way, Orpington BR6 6BT.
Google maps link... 
I suggest we aim to meet at the pub from 08:00am, to depart at 08:10am. It's only 4 or 5 miles up the road to the venue but a couple of years ago there were long queues to get in. But that should give us plenty of time.