Because your weather's been wet, presumably. They hide in damp places in dry weather, and spread out and increase in wet.

One way to cut down on the population is to go out each evening just as the light is fading - this is the time they tend to come out to feed. Spinkle a little salt on each one - it kills them

As Andrew has said, killing them does work - you tend to get the adults then as time goes by you only see junior ones. They breed every 3 months or so, so you can dent the population.

Have found using 2-3 garlic bulbs, crush, boil and put the water and spray all over garden does work. Have only been using for a few weeks but can see less holes already.

When you say 'bulbs' Kildermorie, do you mean whole heads of garlic? And can you smell it when you walk round the garden afterwards?