I use nemaslug in the garden, but it would come expensive on an allotment, especially if you did it at the recommended 6 week intervals, but I do find it cuts the numbers down a lot. The nematodes don't affect anything other wildlife, although apparently they can cut down snail numbers too, so the advice is to keep it away from ponds.
It's worth shopping around, especially taking into consideration postage. Green Gardener does the small pack for £9.99, with no delivery charge.

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