Rik-
the mark 2 Brit arctic bag is HUGE- I'm a 58 inch chest and it fits me with plenty room inside. Ok they were made in the 80's and early 90's but you can still pick them up for £20-30 new. In terms of temp rating, they were designed to be used with mao suits inside aswell as a liner so they are really not arctic bags on their own. I made a fibre pile liner similiar to a buffalo bag to go inside for the really cold weather and it's fine.
Failing that try for a BIG AGNES bag, these are ultra lightweight down bags that are huge.
Woodsmoke