Thanks to all, and I fully understand where you're all coming from, the challenge we have in the UK is that it is difficult if not impossible to find a suitable engineering shop that I would trust to carry out the cut and weld technique, which I completely accept is the best way of removing the cat. Sad but true, maybe I'll go for a complete header replacement.
Thanks guys
Jon
Cutting a hole in the collector part of the header and cutting out the CAT is not that hard for someone in a muffler shop to do for you.
Here is the hole in the collector showing the CAT material to be removed.
Here is the piece I cut out. It was a bit of a chore to peel it out, but it came out and when I cleaned it up I just flattened it back out and welded it back in.
This is as clean as I could get it with the air chisel and a scraper.
Here is the CAT material I removed.
This was done in about an hour total, cut to re-welding the piece back in.
Good luck with yours or just buy a non CAT pipe or aftermarket header pipe.