Right then, for the sake of rounding things off to help others with similar problems in the future, I'm reviving this thread for a final update.

It would appear that the sticky brake problem has been rectified. The culprit? The pads. They were just too tight in the caliper carriers. They've always been tight, though some tighter than others. Didn't matter if they were Bosch, Brembo, cheapo's, I've tried numerous brands. All the same. Seems obvious now, the only reason I'd initially shrugged it off was that only one side seemed to stick. Not both. Surely you shouldn't have to modify parts to fit? Surely, if it was the pads being tight, both sides would stick right? Well, apparently not.

At any rate, I had a set of cheap pads on a shelf in the garage so decided to grind the lobes down slightly with the dremel and whack 'em in. Not ground down so far that they were rattling, just enough that they would slide in and out on the shims with minimal effort. Enough so they could be slid into place by hand. Since doing that I've covered a couple hundred miles and no sticking, no heat, no smell. Nothing. The car feels perkier and I'm pretty sure my mpg has increased slightly as well. So I guess thats it. Not the piston, the slider pins, the shims, the caliper itself, not the corroded brake line or flexi hose, not debris from the road or the dust shields. And not the abs module. Just crap fitting brake pads.