http://www.accommodationz.co.nz/economycalculator.html

gives 14.13 per gallon. I take that to be a UK gallon, which equates to 11.73 per US gallon.

The worst I've ever had out of my M103 or M104 engines is 20 per US gallon, and that was in a W126. That's with mixed and fairly careful driving, but even with city driving you should be getting the 16 mpgUS claimed by MB for the city cycle on the M104. I'd be horrified if I dipped below 20 mpgUK. With no fuel-consumption calculator on board, all my figures are from complete full fill-up records.