rear brake caliper issue

[limax2]

I will re-check but i dont think it will go out anymore without popping out. Should that piston be moving?

welcome to the site Hamish. Yes both pistons should be moving. If you clamp one piston and press the pedal the other should move out. In good condition you should be able to push each piston back in with firm finger pressure. Less firm pressure if the hydraulics are not connected.
The rear caliper pistons on these bikes are well known for sticking if not regularly maintained due to the exposed position below the swinging arm. I do mine once a year, but it doesn't go out on salted road and not in the wet very often. doing what darrsi has just described sounds good  :thumbup .
You might get away with cleaning up the surface of the pistons after pumping them out as far as you dare short of right out. Otherwise it's a case of new seals and as joebloggs says possible new pistons. If it comes to needing new seals make sure you clean the grooves where the seals fit because it is the build up of corrosion behind the seal that can make the piston stiff to return.
A search on here will bring up plenty of advice on doing the job, but as always don't be shy of asking for advice.