In reply to UKC/UKH Gear:
Recently bought one of these and I agree it is light and packs up small. I've taken it out on two overnighters but haven't actually needed to use it yet, so only comments about size and weight so fat!
I googled to find one and bought it from a shop in Lancashire called Tamarack Outdoors. I'm sure there are loads of great shops out there you could get one from (although I had gone to my local GO with their website saying they had them in stock to find, annoyingly, they didn't) but Tamarack were super fast in getting it to me and had a very good price on the filter.