Thanks I've tried that but that side doesn't release, it's a mystery!Try tightening it with the brake applied (use your hand, or you can get a '3rd-hand' tool to clamp it shut).
How tight should that nut that holds the spring be? If I tighten that up it also stops the arm springing back?
Do the calipers spring back when squeezed when they are off the bike? If so I would agree with Fossyant that you are doing the calipers up too tight on the bridge or fork crown. Also, if they are not centred correctly only one side will release.
The tightness of the mount should not influence the tightness of the pivot bearing. Visualise this as a short threaded rod with three adjacent nuts screwed onto it. By tightening any of the end nuts against the middle one (without the middle one turning), nothing happens to the tension between the middle one and the opposite one that's not being turned.