click here for map
NB the arrow is in the right place now I think (Oli).
There is a path / narrow road opp Park Avenue South in Park Road on the left about 200 yards after the swimming pool when heading north. Take the path , on your right is North Middx Club on your left is Calthorpe. Keep to the path & go beyond the next ground on the left & the tennis courts on the right, then turn left toward yet more tennis courts.
The pavilion will be directly in front of you & Highgate's ground on your right.