MOT emission failure, please help

[Marchino]

I just failed the MOT due to over the limits emissions
the local garage here in London told me that can be due to a spark plug...and suggested me to book in a BMW garage fix the problem and come back within 10 days to re-test the car.
The local BMW dealer in London want to charge me £ 144 only for the diagnostic plus any work needed.....

do you have any idea on the cause of the emission failure? I have a ful Milltek but i had no problems with previous MOTs....
The garage also suggested me to try to have a blast on the motorway and then re-test again....
a real nightmare!