Thanks Dnk,
I tried to wiggle the piston out with a variety of tools: rubber strap wrench, nylon strap wrench (oil filter removal tool, basically), mole grips, plumber's adjustable spanner (pipe grips). The strap wrenches just slipped on the piston. The mole grips and plumber's spanner started to score the surface of the piston, despite using some 120 grit sandpaper as a protective barrier. It's only scored the tapered tip of the piston, as that's the only part that's peeking out from the caliper, but I didn't want to continue and knacker the surface.
Having read your post though I guess the tapered tip doesn't matter too much. It's the cylindrical body that needs to be kept free from scratches. How much did you scratch your piston tips by using pipe grips?