BEEVEE, you've sussed it out:- in your words
"....by spacing, do you mean splitting the Caliper to insert a Spacer to cope with thicker Rotor ? Never seen that done before and I dont think thats what you mean, nor would it be legal".
That's how you fit the gp1 vented discs to an Escort, Capri or any other vehicle. The spacer is machined to take another set of O-rings for sealing the fluid passageways across the two halves. Certainly not illegal as many OEM calipers are designed this way to suit vented and solid disc applications within their range eg older 5 series BMW's for example, essentially the same caliper with & without the spacers to suit vented & solid discs. Aftermarket suppliers like HiSpec do this for various caliper fitments too in their current range.
Caliper spacer kits for the M16's come with the spacers, O-rings, longer bolts & usually longer pins to hold the pads.
Obviously this spacer type arrangement is only applicable to fixed calipers & is irrelevant for a sliding caliper.
SFE