I thought I'd post the solution to my problem, in case anyone else ever has the same "fault".
I had decided to run a separate fused supply to the OVP relay, as it wasn't getting it's proper permanent supply, and I couldn't find it's fuse or source.
So I removed the battery to ease feeding the wire through the bulkhead, and trapped underneath the battery was a wire
A wire with a ring terminal that looked like it should be attached to the battery post
A wire that turned out to be the ABS power source
Evidently whoever changed the battery last failed to reconnect the ABS wire, and subsequently the owner had removed the warning bulb to get it through an MoT.
My local dealer had failed to spot it too, hardly surprising though. At least they had the decency to only charge me for 30 minutes work despite spending 90 minutes on it.
On the positive side, I learned quite a lot about my SL
