Hi Mark UK
To me your description of your (rather barbaric! Definitely don’t want to be slicing up newts by mistake!) way of dealing with slugs reinforces the idea that if you create a ‘vacuum’ of slugs others will move in.
I don’t do much to deter slugs and don’t seem to have anything like the problems it sounds as if you have. Nature has a way of sorting these things out if not interfered with too much.
I think you will find that it’s widely accepted that the large slugs are useful in the garden and that if they didn’t exist we would have much more slowly rotting plant material to deal with. Quote from Carol Klein (link below): “Quite honestly, if we didn’t have slugs and snails we would all be knee-deep in debris and rubbish because that is what they do: they chomp through it.”
Maybe some frogs in the garden would help the hedgehogs. Or apparently ducks are good (if your garden is a bit larger).