Thanks for the replies.
Finally got the downpipe off with the help of my angle grinder! - trouble is the supporting bracket from the back box came away as I released the pipe away.
I'm now liking the idea of removing the whole system as suggested - however looking at what's left the nuts connecting the cat to the back box its not going to be easy.
What I'd ideally like do is replace it all with stainless steel - just don't think I'll be allowed