Ah the Panhard rod & it's mysterious bracket.
I said, did I not that it didn't look like a Jeremy Phillips design? Turns out I was right, JP mounted the Panhard rod to the outboard lower corner of the rear bulkhead in a rather elegant way that does the job & crucially - doesn't dig holes in the road. Today I've cut the angle off the fork, re-drilled the bolt hole an inch up & cut an inch off the bottom, tomorrow my brother will weld the angle back on to match the new position.
It's a simple fix - possibly too simple, but hopefully the car will now hit the bump stops before the bracket hits the road. I have a back up plan if it's still a problem, it's a more elegant solution but involves more complication, I may do it at a later date even if this works, as it'd lighter & stronger, but needs the car off the road for longer.