In reply to Gordon: This rucksack is first rate. I used mine all last winter from days out walking in snowdnia to grade V in Scotland. No gimmicky extras that are surplus to requirements and very stable on the back.
As for the extension, again nothing fancy, simply tensioning straps on the lid that gives the extra space required to get your rope inside the pack when its full.
Sizing - I am 5'11" and use a size 2.
I think that rock and run have this sack in their sale or if you are interested I can get 35% off the normal price of £100. If you are interested please E-mail.