Waterproofing a basement around Clapham Common is always a challenge thanks to the heavy clay soil and high water table. When water is actively pushing through drypack underpinning, standard tanking won't hold up on its own.
The trick is fixing active leaks before laying down protection.
First, hit running leaks and weak joints in the drypack underpinning with a quick-setting waterproof mortar to plug active flow instantly. Follow that with a coats of Hydradry Slurry brushed across the underpinning to coat the masonry and hold back hydrostatic pressure.
Once the basement is watertight, install a full Type C Cavity Drain system over the walls and floor.