In reply to The Ex-Engineer:
Ah, OK, mainly my mistake - I was thinking that the "Lite" Rab was just pertex, and the normal survival zone was coated. Which I have to say didn't entirely make sense as I'm not sure what benefit a pertex bivi bag would add.
So, pretty much the system I've used, and in the same kind of conditions - down bag with normal pertex outer, rab survival zone over the top. It's been perfectly serviceable for camping in wet/cold weather, and for snow holing. Haven't used it outside of some kind of shelter. No obvious problems I can remember!