I think you can go to your Ford agency parts department and they can show you the various types of muffler systems available from them.The least expensive solution would be to purchase a high quality performance muffler from a parts retailer and then find a shop that will cut off the original muffler and weld on the replacement and a nice stainless steel finnisher. Don't expect it to provide you much performance improvement. Several brands like Borla, Dynomax and Thrush are known to offer fitments for Fiesta in America. I imagine that at least one of them also sell in the U.K. if you need a part number for the Borla muffler for the Fiesta I can find the box mine came in and post it here. It's a stainless steel item that will last literally forever.