Howard Garrett (The Dirt Doctor) just this morning on his radio program said he recommends the pickling vinegar, at 10%, for the strength of vinegar used to kill weeds. I have a 20% jug, so I'll water it down and it will last longer. He also talks about using a surfactant (soap) at the rate of about 1 tablespoon (1/2 ounce--does either of these measures mean anything in the UK?) to the gallon of vinegar. A gallon of vinegar will cover a LOT of territory.
Slugs are wonderfully easy to kill if you don't mind sharing your beer. I dig a hole and sink a plastic margarine tub about half-way into the ground (keeps it from tipping if it's windy) and put an inch or two of beer in it. In the morning I dump out the beer and dead slugs and snails. I move this around the garden and do it as needed. Usually when the plants are tender young things and have just been put in the ground.