Stuck between a S and M for a super thermo t-shirt.
I'm a M in most UK brands (ME, Berghaus etc.), but I've heard it stretches and can be a little on the loose size.
I'm 6', slim, went with medium for the superthermo. Small for the artic double, has been fine. Definitely wouldn't want any bigger on the artic double.
They're good stuff. Just be prepared for the fishnet comments.
FWIW I bought the long sleeved thing with the reinforced shoulders and found the S far too tight. The M works fine. It's supposed to be tight though. I'm a S in practically everything else.