Help to ID this bug/pest?

I've grown kale successfully for the past 2 years from seeds but this year the kale that my Dad brought as seedlings/very young plants several weeks back have failed to really do much. At first I put it down to slugs. Although they only nibbled a few leaves, I thought it might have shocked the plants somehow. However, today I was pulling up some of the worst specimens and found in the soil directly underneath them some suspicious looking characters - I'm pretty sure they aren't worms and only 2 of them made any attempt to move around. Could something have laid its eggs in the soil back at the garden centre where we got them?