I had the same thing with mine recently, same side. I did what is suggested above to free it off, ie. push it back and then pump it out a few times (I put a bit of brake fluid inside the seal around the piston as well) but that was just a temporary fix until I could replace the caliper. I got a second hand one from autolink, which was 'tested and guaranteed good' for ?30 plus delivery and VAT, I fitted it in about 15 minutes and the job's a good 'un! As the previous poster says, more than likely the reason it's sticking will be because the piston is pitted and has rusted, you can replace the piston and seals (I've seen a guide somewhere, it's definitely in the Rod Grainger book) for about ?30 quid or so but you'd be as well off with a second hand caliper. You can also get a remanufactured one from autolink for about ?70 plus your old one.