My girlfriends housemate is growing some veg in their paved courtyard / garden - all in pots.
They are having a really hard time with slugs and snails who seem to be able to strip a healthy lettuce down to the ground in one sitting!
She bought some slug barrier tape that is designed to go around the top of a pot. From what I could see it has copper in it, and a static electric charge.
Looked very gimicky to me, and perhaps is not very useful for an allotment, but I shall watch with interest and report back if it does the job.
anyone else tried it?