OK, sussed it.
You need to undo the bolts at the top and bottom of the caliper using a 13mm spanner and a 15mm spanner to hold the slider pin in place while you undo it.
Took both rear calipers off and had a go with a wire brush and high pressure airline, before regreasing the sliders and greasing the faces that the brake pads slide against (no, not the braking surface...)
Backed off the piston a couple of turns and all seems as good as new... that is the brakes work and there's no nasty grinding noises when you spin either of the back wheels.