vaux corsa fuel mpg issues?

hi.
my girlfriend has a 1L p reg 65.000 mile corsa.its mpg seems a bit disapointing say between 25-35 max mpg...

plus engine seems to run fine most of the time except periodicly it missfires and wont tick over if you let it idle and the yellow engine management light comes on at the same time. it might do that for aprox 60 secs n it will just sort itself out although the light stays on.

are the mpg and missfire related and what should i look for to sort at least the mpg issues?