They will, or should, have measured the thickness of the discs. Ford stipulate the minimum thickness. If you know the figure, from a Ford dealer or Haynes Manual, you can use a micrometer to check them yourself. 40,000 miles sounds about right anyway and as said it is always linked to your driving style. If you can, buy Ferodo discs and pads as these will equal or better the braking performance when the car was new.