One of the pistons in my rear SLX brake calliper decided it wanted to stick up recently causing the lever to feel bloody awful.
Anyway, when I got it on the workstand I took the brake off and pads out and when I pulled the lever a couple of times the stuck piston still decided not to move and the other popped right out of its housing. Bit of an error and I got mineral oil all over the place!
What I want to know is, does it sound like the calliper is broken? I was planning on taking it apart when I get the time and having a look at the internals. Any advice?

Nah, just sticky, not uncommon. Put the other piston back in, rebleed, secure the "loose" piston so that it can't move, then when you pump it the sticky one should move. Once it's out, thoroughly clean it and the seal, and the "gutter" in the caliper where the seal goes, then carefully reassemble and see if it still sticks.

It might tbh, and the drawback of Shimano is it's hard for mere mortals to buy individual parts. No bother though, when one of my XT pistons went sticky I bought a brand-new XTR caliper for £15 off ebay complete with pads and swapped 'em 😉