The power of advertising

I put B's old Punto on the roadside this morning. Four phone calls later and it has sold. After dithering and prevaricating we finally went for the deal that we sheepishly rescinded on back in December.
But should 12 months MOT have an apostrophe? At first I thought it should and then I thought it should be after the S and finally decided that what it is actually saying is, MOT for 12 months - which is what I should have written in the first place.