Well, it's taken a while, but I'd like to report that my mate finally sorted out the original problems with the mod wiring.
To refresh everyone's memory, I enlisted a friend to do DavePlaysBass' mod to enable switching between Single Coil (outside) / Parallel / Single Coil (inside). However, something went weird with the mod. Read the OP for more details.
I now LOOOOOOOOVE this bass. I had never quite gelled with either series or parallel modes for most songs/styles I play. Now with the mod installed, I am very happy.
I played the bass in church last Sunday and it gave me pretty convincing P-Bass tones (SC outside, neck pickup), J tones (SC outside, both pickups or back pickup) and something like a punchy, ringy Music Man tone (SC inside, both). Awe. Some. The SC modes are currently my favourite settings.
For the record, the problem with the mod was that one pair of grounds had been soldered to the wrong place (the bass tone pot, I imagine).
So very glad to have this bass back, and couldn't be happier.
Steve.