Not sure, best time is never really but in extreme sluggy conditions I would use a very few, say one per plant, and that could well be now as slugs are more active in warm weather, also they are breeding now to leave slug eggs overwinter. I prefer the night time foray with a torch but appreciate this is difficult at allotments. If pellets are placed under something damp, one per station, so birds cannot eat them, then dead slugs are collected in the morning for the same reason, their population will be reduced. Keeping edges tidy with no overgrown areas near tender seedlings is a major help, as you will have noticed. Bark is sometimes a shelter for slugs.