Small white snails
I would suggest you try Advanced Slug Killer pellets. These pellets also kill snails but are safe to wildlife like Hedgehogs. The pellets do not contain poison like traditional pellets but do contain iron, which stops the slugs and snails feeding and kills them. The pellets also breakdown in the soil giving the soil a boost of nutrient. Sprinkle them over the areas where you have the problem. For larger areas it would be better to use the larger pack size. The code for the product on the Harrod Horticultural website is GPC-197 for a 750g unit. I think you will find these are a safe effective way of reducing the snail population.