Not keen on the look of that Buffalo bag - it seems a bit more close fitting on the head than I'd like. I prefer to have that adjustable. May try it in the flesh next time I'm in Cotswolds for future reference though.
I ended up going for the Rab Ascent 700 having seen it in the flesh - it really does loft well. I suspect I'd be happy with either the Titan or the Ascent but I've got a bit of Rab stuff and it's all been top quality, and despite the fact it's just going to go in the airing cupboard for the next month I couldn't resist getting it there and then rather than waiting.
I found a place online selling it for £168 after a bit of digging and got Go Outdoors to price match +10%, so paid £151. Very happy at that price - I'm sure it'll be cheaper in the spring, but right now that's well below the average price.
That being said, for anyone interested in the ME Titan 650 Nevissport have it cheaper than that link above - they're doing it for £153 delivered:
http://www.nevisport.com/pr/5630/titan-650-sleeping-bagThanks for the help everyone.