It's the Laufbursche Pack Sack Plus. I got it direct from them in Germany, not sure if they sell through anyone else. Great pack. I've added some shock-cord to hold a sit pad, and yesterday added some more shock cord to the front (maybe the back, I'm never sure which is which on a backpack) to hold a tarp/tent outer etc.
Thanks Suggdozer. Think that's about as light as I can manage at the moment while still being comfortable and safe, and think the next time I go I'll be a bit heavier, but was a more than worthwhile experiment. Thanks for watching