One of the pistons on my rear slx disc isn't moving as freely as it should do. I've tried bleeding it but to no avail. Any suggestions as to the best course of action to free it up?

A quick fix- remove wheel and pads, thoroughly clean exposed pistons and pad housing, drip a small amount of mineral fluid around the pistons and work them in and out(don't pump them out too far or you will regret it).

Following further investigation it would appear that the IS to post mount rear adaptor is too wide. E.g. its pulling the caliper off centre even at it's furthest edge of adjustment. So despite my best efforts one piston is pushing the disc to meet the other piston for braking.

Anyone ever filed down or shaved an adaptor? I'm assuming it'll be fine taking 1-1.5mms off of it to help centre the caliper on the disc.

First, and most obvious, is your wheel fully seated in the dropouts?If so, then it depends on which way the clearance lies as to how you tackle the problem, but i'd suggest getting the IS mount faced back so you know everything's square to start with, this should increase the clearance on the outer side, you can then add shims if it needs to go in the other direction.(I have sometimes had to file caliper slots and adapters, usually on cheaper brakes but never on Shimano.I'd suspect the frame mounts.)