Just thought I'd close the loop on this after my inspection on the weekend. Big E was right. The bulkhead was not sitting completely flat on the chassis. This was due to some extra material on the left hand side bulkhead mount. Once I shaved this off with a scalpel blade, the bulkhead sits completely flat.
It's worth noting that we're only talking about a few thou here. However a few thou at the bulkhead will be amplified many times at the end of the shock tower where the shock mounts. Any disparity will result in offset droop.
I've actually seen a few other cars since with this issue. If you haven't checked it, I'd take a look.
Whilst on the subject, I also took the time to check the front droop, which was also out, relative to the rear. I did this using a set-up board, chassis blocks and measuring the axle height with an AE ride height gauge. I don't know for sure, but I suspect this is being caused by a slight imperfection in how the front skid plate mounts up to the chassis. Again, a very minor imperfection here will be amplified many times at the axle.
The only way to compensate for this is to wind one of the shock ends out so the axle heights are even.
Now I know my car is completely tweak free. At least until the next crash
Cheers,
Scotty P.