that train will be rammed, so turn up early for the first one.
When you get to Clapham Junction you should be able to follow the crowd, but, just in case....
go out of the station at the barrier close to platform 17. Watford trains come in to platforms 2 or 17.
Go straight through the concourse and you'll see a Caffe Nero. Go out on to the street and go to your left, but at the Arding and Hobbs traffic lights just a little way down the road, go right
Having turned right go on to the first crossroads and turn left at the traffic lights up a bit of a rise.
When you get in to the one way system, filter right, and , having turned, just go straight across the common. Traffic in the oneway system can be a bit rapid, so keep your wits about you.