BTW... in the US I don't think BMW replaces spark plugs before 100k miles. Aren't they suppose to last 100K or 7 years for the federal emission warranty?
It IS a good idea to replace them sooner though. My tuner said I should replace mine at 50K kms if you run a tune.
EDIT NOTE: Yes, on Turbo cars the spark plugs get changed at 45K miles. NA cars get the plugs changed at 100K.