In reply to Tree:
> ...small black bags like the kind some toner cartridges come in, and a petrol can to pee in- you hold the can, your mate holds the bag...
No need for a mate.
1. Get a plastic bag that has loop handles (e.g. a freezer bag) and put some kitchen roll in the bottom of it.
2. Thread a sling through the loop handles.
3. Drop your kecks.
4. Crouch down slightly, then lean back against a vertical surface (rock, tree, etc.)
5. Loop sling around your thighs using a carabiner. Arrange such that the bag is under your anus. If the bag is big enough - e.g. a shopping bag - it will fit around your entire arse. Unless you have a particularly large arse, of course.
6. Get your Lucozade bottle and stick your pee-pee in the end (sorry girls, you'll have to experiment with funnels and whatnot).
7. Do the business.
8. Dispose of toilet paper in the bag.
9. Tie off the bag as low down as possible (i.e. enclosing the waste in as small a bag-space as possible).
10. Turn the remaining open part of the bag inside out and tie again, so that the waste is inside two layers of bag.
11. Pack the waste bag inside another bag to avoid the urge to buy a new rucksack.
12. Dispose of as one would a dirty nappy.
13. Place Lucozade bottle in your mate's rucksack.