Well, just for closure for the curious ... I was passing the workshop where this car has been sat, and they finally had authority to strip the engine.
It turns out the crank sensor picks up on a "pressed on" toothed ring onto the crank itself inside the engine, rather than the flywheel. Hence why unable to diagnose further without stripping, access via the starter motor would not achieve anything.
This toothed ring has indeed warped, or been subject to previous mischief, as the engine looks like it has been apart recently. It appears to be only held in place by the interference fit, although I did not see any key-way or pins, it may have utilised some beyond the area I was able to view briefly.
As there was no loose metal objects or anything that could "foul" the ring, the only plausible thing I can think of would be excess heat causing it to buckle and warp, or back to prior bad workmanship. As the car came in as a non-runner and the owner being at best vague about the failure, there's not much more to say.
I did take a couple of photo's but looking at them, they don't really show clearly the issue, so posting some pictures of oily engine parts is rather pointless as we've all seen them before!