+1 for LOMO bags - they're superb value.
I have a
40L dry bag (not the motorcycle specific version which has webbing loops). The standard bag is �8 and is the perfect size for camping gear and stuff that you want to get out all in one go.
I also have one of their
40L roll top dry holdalls, which is bloody brilliant for �21. Ideal for shorter trips when I don't want to take hard cases. Much easier to find individual bits and pieces in it as opposed to an end opening dry bag where you tend to find the thing you want is always buried down at the wrong end!
BTW - They also do 60L versions of both the above bags, but they are massive and a bit unwieldy.