This was the worst pad by far (the rest had 1-2mm left), and was only down to the metal backing plate on a small patch - maybe half the size of a postage stamp. The rest of this pad had the thinnest smear of friction material left.
The disc was verrrrry slightly marked, but nothing I'd worry about. Could barely feel it when running a nail over it, so I was happy to stick new pads back on.
Slide pins were moving lovely and freely, calipers wound back smoothly too so nothing suspicious there.
Judging by the way this pad was stuck, I think it must have over-traveled in the guides/slides (due to being very worn/thin to start with) and got jammed in against the disc, then got thoroughly stuck there...
Nothing a big of brute force on my part couldn't sort out though!
Guides/slides came out and were treated to lashings of brakd cleaner and wire brush action until they were shiny again.
New Pagid pads, a smear of Ceratec grease on the guides and rear of the pads, and all back together.
Stops nicely, pedal feels good, handbrake is at a nice height, and no noises at all.
Got lucky there was no damage this time, I think I'll be doing her car for her more often!