Started by thinking of new galvanised chassis.
OK, they have given it a good look over and I only need the rear quarter replaced, despite a welded ARB bracket.
Should be doing it late next week, or the week after.
And a few other bits, then a...
Just to update this topic for posterity in case anyone reads this in future with similar issues.
After some heavy discussion with Bullevard, they finally agreed to test the full exhaust system by sending it to Roetfilter-Centrum who found that...