Hi Andy
I have the S Lab5 set. As far as fit is concerned, unless your a small bloke the M/L is probably best. If your very large across chest, then I doubt the pack will fit. The sizing is based on Salomon clothing. Bear in find, these packs wear slightly different from "normal" backpacks in that the fit is more akin to doning a waistcoat. They are phenomenomally comfy & stable over long distances, very well made and designed. In fact, I doubt I would be happy going back to the more traditional style backpacks.
Finally, availability .. well you've probably got more chance of finding the loch ness monster than an S Lab12 set in stock!!! Very limited supply from Salomon in the UK, & when stock does arrive, its straight back out of the door. But well worth waiting for. If you do find a UK supplier with stock, please let me know, I've only been waiting for the last 3 months 