Anyone got any ideas how to get this piston to move, it's stuck solid

 

try pushing it back in and work it back and forward

 

Tried that with a g clamp, still don't want to move

 

I have an old scuba cylinder in the garage I use for these problems ~200 Bar (2,900PSI) of pressure frees them almost instantly but will shatter the wood if you're not careful and you will get covered in any brake fluid/grease residue

People who have precharged reservoir style air rifles would also be able to help you out as their cylinders would also be of a suitable pressure and would have a suitable hose

Might be worth asking a local garage to load the caliper with compressed air to free it for you.

 

I removed my caliper then reconnected it to the brake line and used G clamps to lock the good pistons then jumped on the brake pedal to release the piston.