Fill your car until the fuel pump clicks off once - don't squeeze that last bit in, just stop on the last click.
Reset the trip.
Now do some driving.
Now re-brim the tank, using the same procedure as last time, preferably at the same pump as last time (easy on the Island).
Check the litres dispensed on the pump.
Check the trip.
Now you need to get your phone out and do a quick calculation to get from litres, to gallons, to MPG.
litres/4.546 = gallons
miles driven/gallons used = MPG
So for my last tank:
51/4.546 = 11.22 gallons
287/11.22 = 25.58MPG
I was surprised to find no MFA in the Polo, too. You just have to accept you don't really need an MFA and that you just like to have one.