Hello - I thought it time I made an update in here. A family visit to the south of England
finally made it possible for me to take the car in to GTT and, I can quite happily say that:
My GTT Spiroslots are fantastic and problem-free!!
Roland drove me out in my car and demonstrated a few emergency stops and things to look out for when checking the brakes for problems. Amongst other things, I got a lesson on driving with high performance brakes - the trick is to be a bit more relaxed when applying them, and actively control the car to a stop rather than just gripping the wheel straight and throwing out the anchor.

Thanks Roland!
As for my callipers, Roland knew exactly which rubber seals the other garage here in Scotland had been referring to and it's not problematic as they had made it out to be.

So that's not really something I need to worry about.
And finally, my braking fluid was uprated to 5.1 - this makes a heck of a difference!!! The winding country road on the way home consists of some very straight national speed-limit bits interspersed by some 90 degree corners. Previously, when I'd use the brakes heavily and repeatedly on these sections, by the time I got to the end of the road, my brake pedal was all spongy and would sometimes sink further than normal on the last corners. A few pumps would make this problem go away but after it happened the first time, I never quite felt the brakes had the same feel as before. Now that Roland has uprated the brake fluid, the feel of the brakes is much more solid and extremely consistent - regardless of how many applications.
Anyway, as aforementioned in the post above, I'm not experiencing squeal when the brakes are not in use (this was due to an incorrect initial installation in Scotland). Also, now that a few more hundred miles have been put on the pads and discs they seem to have settled in even better than before: the low-speed squeal I sometimes get around town areas (while never quite fully disappears) is now dramatically reduced. One thing I noticed is that it rarely did it whilst I was down south (nice warm weather) so I think the brisk climate of Scotland might have something to do with it.

After a spirited drive and the brakes are warmed up (well,
hot - with the way I sometimes use them

), they're virtually silent.
Concluding thoughts: The added stopping ability the GTT Spiroslots add to my MINI is phenomenal! I highly recommend them - but with one note of caution: get them fitted by GTT themselves so that you're absolutely sure they've been competently installed. If I'd done this initially, I would have avoided all the problems I've experienced with the initial incorrect installation of the Spiroslots.
And that basically concludes my review.

Any questions?
All the best,
Andrew.