More information about Watling Gardens and a map can be found here.
Walk 3 minutes up Shoot-up Hill after turning left out of Kilburn Tube Station. The Watling Gardens Estate is on the left, almost opposite St. Cuthbert's Road. Or you can travel by any of these buses:
Walk on to the estate, through the entrance from where you can see the little playground, then walk past the playground:
Go to the left of the block with the white panel and a yellow sign showing "WATLING GARDENS 97/135:
Just beyond and to the left of that black notice:
Walk just beyond the small black notice on the LEFT of the picture:
Now you can see the door.
And you can see the sign (though it doesn't show up so well when it's dark); open the door and come in:
And the room looks like this: