Baal writes
Put a small pond on my allotment but no frogs last year or this! Want to
kill those snails/slugs without chemicals!

Apart from stealing frogs, any ideas on how I can encourage them?

You need to provide acceptable habitat around the pond - basically damp
foliage area for foraging, ideally not too isolated from other frog
habitats. Might be difficult in an allotment environment.
--
Kay