Damp Penetrating Party Wall of Extension

[Hanwellandy]

We have an extension, built around 1988, with a damp problem in one corner on a party wall. This corresponds with the zone adjoining next door's patio, which is about 300mm above our floor level. It is also where next door has a drain, in poor condition, which I have tried to repoint a bit and dowsed with waterproofer. After messing about with Dryrods etc, which didn't help much, I decided to remove a brick; this revealed that whoever built the wall did foresee the problem and put a bitumen DPC between the 2 layers of brick - see photo. This is now in poor condition (or maybe was rubbish to start with), which is presumably why damp is getting through. Might there be a way to inject a better DPC between the two layers of brickwork? I was thinking of squirting some expandable foam in, or maybe some tanking slurry. There doesn't seem to be a better way, other than removing quite a lot of bricks and applying the tanking direct.