In reply to Baron Weasel:
A slight aside but if you have an overhand in your system an it's likely to get weighted, so becoming hard to undo, you can clip a spare karabiner round one of the pairs of strands forming the knot when you tie it. When the knot is weighted it'll tighten on the 'biner which you remove before undoing the knot. Just don't clip anything critical into the karabiner.