Products that contain Bentonite are what you need Les
We used plugs called Bentonite water stop plugs to fill the holes left when shuttering was removed after a concrete pour if the pressures required restraints had to penetrate through the concrete walls (obviously leading to leaks below ground) you'll often see these in underground car parks
In your case, trace the leak point(s) overdrill to the plugs recommended size, shove in & bingo the Bentonite will swell and seal, there are also bentonite mortars you can mix and plug with, ordinary sand/cement is useless
ps, as used on the Channel Tunnel
