Thanks everyone for the replies. I put a new muffler on yesterday and
the old one had a ceramic honeycomb material in it. I thought this was a
part of the muffler that had broken down. 

After reading the posts about getting a new cat I researched them and
found they contain a honeycombed ceramic element. I gas this had broken
down and worked its way into the muffler so it looks like a new cat will
be the answer.

Anyone know how this was affecting performance?