rear caliper

[solorider]

<edited due to insufficient research and gross stupidity>

If you can whip them out (and it's a big if) without damaging them, you'll find the rust is from the brake pad backing plates and might clean off without too much problem. Your bigger problem will be getting them out though. Judging by the state of them, they'll be stuck well in there. I'd take the pads out and stick a block of wood between the pistons, then pump the brake until they're almost popping out.

amazingly the rear brake works perfectly, it is not sticking or binding in any way and the bike was in the same condition as last year and it sailed through an MOT.