Christchurch Avenue, Behind the leisure centre car park.  Just walk through the car park to the back, and you'll see it, if you haven't already heard it.

The leisure Centre is about 15 minutes ride from Harrow On The Hill Tube station (Met line) and about 5 minutes from Harrow and Wealdstone mainline station (Bakerloo and Silverlink).

If you are lazy and don't have a bike, get the H10 bus which stops right outside the leisure centre from Harrow town centre.  Any problems finding it, just ask, there's always people about who know where it is...