I note you're in Scotland - most of the UK has been incredibly wet this summer, not so sure about where you are, but this has meant an explosion in the snail and slug population. Slug pellets - the little blue pellets of death, metaldehyde based, will do the trick, but don't over apply, just a light sprinkle around the day lillies, reapplied after rain, or after a couple of days if they've all disappeared. Day lillies are attractive because they trap moisture in the base of their clumps of leaves, so its a useful place for slugs and snails to lurk so they don't dry out and can remain hidden. The previous answer mentions copper tape - this can be quite effective most years, but as there are so many this year, its not as efficient currently. You could try both. There's also a product called Slug Gone, made of pelleted sheeps wool which you spread around your plants. Its expensive, and from anecdotal evidence this summer, once its sodden, it acts like a welcome mat rather than a repellant, but works quite well when the weather is dry.