I now have full size spares. One 17" factory alloy from a wreck fits the MC zetec diesel and one 16" steel from a focus fits the MC LX diesel .
The wrecker was OK to sell one alloy wheel because the others were damaged.
The 16" almost fits the spare well (1cm) it's within a mm of it's life.
The 17" alloy is about 5cm high, being near new.
Re the MD, Google tells me the disc/caliper clearance is designated the 'X factor',
which sounds about right:
https://en.wikipedia.org/wiki/Wheel_sizing#X-factor