I have finally given up with repairing the rear exhaust boxes on my Jaguar XJ6 X300 and have purchased the rear four boxes in stainless steel. I bought them from G.E.S exhausts for £510 including delivery and VAT. As I have said in one of my previous articles about the cracked down pipe, the first sections of the exhaust contain the catalytic converters and I believe they are made from stainless steel (the boxes, not the catalytic converters).