We have a happy B7 again!
It went back to the garage today for inspection, and it turns out the mechanic had already cleaned and inspected the caliper when the job was first done. The slide pins were still moving freely, the piston wound back and returned without trouble, but even after a second cleaning and reassembling it was still binding on both sides. From what I could gather, the handbrake mechanism on the calipers themselves were not fully letting go, which wasn't a problem on the old worn thin discs and pads, but with the brand new fat discs and pads it just wouldn't release enough to spin fully freely. Two new callipers and it's driving like a dream again, took it out for a good blast to celebrate. It brakes so much better now, so I'm wondering if the old calipers were actually doing much at all...
Just the xenons to go now and then I'm putting the saloon on a diet from my wallet for a while