The Invalid wrote:
Several of my young cabbage plants have been nipped off right at their
bases.
What's doing this? Ants, leatherjackets?
Its damned annoying

It's slugs if it's been eaten at the base and and white moth if there's
holes on the leaves. Look for companion planting to deter moth and
aphids (I use lots of nasturtiums) and I use plastic bottles which I
ring with copper tape to deter the slugs.