ave mpg readout

Just noticed that my 2010 1.8tsi SE Estate (not maxidot) both the ave mpg and current mpg figures are reading completely randomly when moving. When stationery with engine turned off the ave mpg figure just shows three dashes and a slashed zero. All other readings appear normal.

Being an old codger who,  for no real reason at all,  habitually notes fuel bought and mileage in a book I don't look at the ave mpg readout very much or take any notice of it.  Just wondering if the cause is something to do with a flow sensor or a maybe a software fault? TBH don't want to spend a lot fixing it if it's unnecessary . 

Thanks for any ideas,