Something like this might work better than the 197/200 diffuser. It's much shorter, and also shallower...meaning you possibly won't need to cut out the wheel well!
http://cgi.ebay.co.uk/Mk6-Fiesta-ST...arts_Vehicles_CarParts_SM&hash=item45fba700df
something like this would fit much easier imo, no chopping of wheel well as the Fiesta has one.
http://cgi.ebay.co.uk/Mk6-Fiesta-ST-rear-valance-diffuser-splitter-/300562862696#ht_915wt_932
Something like this might work better than the 197/200 diffuser. It's much shorter, and also shallower...meaning you possibly won't need to cut out the wheel well!
http://cgi.ebay.co.uk/Mk6-Fiesta-ST...arts_Vehicles_CarParts_SM&hash=item45fba700df
get some 197 wing vents in it too