In reply to JayPee630:
My Rab Strata (Polartec Alpha) fits over a microfleece, itself over a mesh baselayer, and under a Montane softshell or ME hardshell according to the weather. All sized medium. It's a fantastically flexible system which all but does away with the need for a belay jacket. In fact I have both the long and short sleeved versions, used according to conditions, or layered together. You can't believe how breathable this stuff is until you try it.
My wife has the Atom LT, which looks great but it simply doesn't breathe as well away from the underarms.
Haven't tried the Montane Alpha guide but I assume that will work extremely well too. It's really an Alpha gilet with powerstretch (or similar) sleeves so not quite the same thing.