Around town my fiesta st did about 26 mpg average across a whole tank. Would do 41 mpg if whole tank was on the motorway. If you drive like a dick then late teens.

My old puma engined zetec s would never worse 30mpg across a whole tank not mater how hard you killed it.
Was also quicker than the standard st.

Early fiesta sts are worse on fuel for some reason ( per face lift) and decanting them makes them appalling.

182 are really shit on fuel for town driving.

At 350 miles a week you should be looking a deisel really. Perhaps a seat Ibiza cupra pd. 160bhp and comes with ap racing 4 pots as std.