Firstly the head unit is absolute quality in every way including sound quality and it is something to rave about Pete! ;)
yes a faulty O2 sensor can cause poor mpg but not really when it's the sensor after the CAT that monitors emissions. The light is off at the moment anyway. Hasn't come back on in over a 1000 miles now.
Hardly use m mode, always just stick it in D and go.
No Stuart I sold my 200, only up for sale for a week and it went :) and no, no modifications now to this one, keeping it standard.