Thanks for the replies!
Bago - yes, I have lots of those snails too, but pond snails live in the water and are no threat to your vegetables
Ponds are a really good thing to have - they encourage newts, frogs etc - I spend ages looking in my old bathtub now.
Ebay - mmm but what else might I be tempted by?
I don't know where my nearest wild pond is - the one at Heeley City Farm has dried up but not before I "rescued" some daphnia type creatures
. While I do want the snails to control the algae, I also want them to create a balanced natural pond (so no fish) that will more or less look after itself. And I'd love to have leeches as well - some types carry their babies around with them!
Maybe I just need a bit more patience