Lock Gardens Safe House - Noel Road, Islington - Tinker Tailor Location
The climax of Tinker, Tailor, Soldier Spy and the revelation of the mole take place in the Witchcraft safe house at 5 Lock Gardens.
Lock Gardens does not actually exist as a road and none of the Camden locks were considered suitable locations for shooting the scenes described in the book. The BBC chose City Road Lock in Islington and nearby Noel Road as the location. We can be sure of the Noel Road location by comparing this shot of Smiley on the doorstep with a photo looking down the same road today.
It's harder to be so precise about the actual house in question, but I think it was probably 70 or 74 Noel Road. A location visit will probably be necessary to be more certain, but I'll let you compare the architecture in the photos below. The first image is obviously from Tinker, Tailor and the others show the exterior of the houses today.
Finally here are comparison photos of City Road Lock as it appeared in the series and as it is today.