In reply to MtnGeekUK:
hehe time for a freezer bag detour:
Once some uk supermarket freezer bags were fairly decent. The current zip slider ones from supermarkets all seem to pop open when being placed in a pack or rolled and tucked under a shoulder strap etc. They all tend to be an awkward shape for map purposes and the larger ones often have flat/stand and fill bottoms. I haven't tried them all but I tried a lot with a friend recently helping him pack for a trip when we had no Ziplocs on hand other than small. The press to close ones are all single seal and inadequate. There is one UK brand that is ok called Lakeland. I think they are only available online and from their own stores these days.
Even proper Ziploc(TM) push to seals(double) won't take that much abrasion and they are the only food type seal bags I use now. The corners will wear after a few passes across something. I use them a lot and they are probably the toughest and most reliable but currently a PITA to get hold of in the UK unless someone has a costco card. I order online in bulk instead. Also the design changed recently and included easy separation tabs and there have been many reports of these bags failing not on the top but at the side seam so I would only recommend the older ones without the tab until that is fixed.
As an aside, I noticed ClasOhlson had less than half price OpSaks in store the other day. Not sure if all sizes but I stocked up on the smart phone size 3 packs. They are expensive for a zip baggy but they are durable. The ones I got were only about £1/ea.