I'd had the ABS light on for a bit, and got around to trying to fix it today.
Reflowed the big solder joints and it seems to have sorted it.
The box had previously been opened (presumably for the same repair) and the previous repairer had cut a small section of the housing away and resealed, which looks like it removes the need to mess with the factory seal around the edge of the unit or the removal of the posts the screws run through. Maybe a good option for people who don't want to crack that original seal.