Well last night I came up with a cunning plan
I found a torch,armed it with freshly charged batteries and placed it where it could be found without emptying more than four drawers.
I then waited till it was dark
When the sun had set and darkness was all around I picked up the torch and switched it on.A bright light emitted from the end

so I aimed this source of illumination through the window and surveyed the garden.
There lurking beneath the bird feeders was..............a Hedgehog
First we've seen here in a few years (not even any flat ones

).
I'm assuming this will be responsible for the snail slayings but these are taking place at the side of the garden and it would be very difficult to stake that area out without the culprit eyeballing me (plus I do need my sleep you know

)
A happy shenk1