What with lack of space and other priorities I have pretty well given up sowing our own brassicas and rely instead on the excellent www.organicplants.co.uk to supply me with well-grown plugs. The important thing with any plugs is to get them planted out or potted up as soon as possible.
I’m potting them up because that way they will be stronger, larger plants when they finally make it into the ground and in the meantime I can protect them from the predations of slugs and snails and cover them with enviromesh to keep the cabbage white butterflies away.
When I’ve got lots of plants to pot up I fill the wheelbarrow with compost and park it in a shady part of the garden (not difficult in a garden full of trees) and do my potting up there. Bending over a wheelbarrow can put quite a strain on the back, so in my hillside garden I position the wheelbarrow on the edge of the lawn which is raised 50cm above the adjoining path and can work at a comfortable height – alternatively sitting on a chair or stool will also save your back.