I called back into the renault garage to ask them why they fitted the twin CAT to my MK2 172. They checked and then told me the old cat had been superseeded. Not sure if i believe this or not. BUt the price for the CAT they fitted to my car was £761.28+vat, the single CAT is just ver £400. I got them to print me a copy of the invoice out.
here are the details from the invoice
go replace cat converter 1393 £29.68
downpipe Exh C3 6000073701 £730.00
gasket f/pipe GA 7700823540 £1.60