Sori, @henddraig: For some reason I didn;t see a notification of this.
Anyway, I get my nematodes from www.GreenGardner.co.uk They come in a package through the post that you con keep in the fridge (it is well sealed!) for a fortnight, I think. They come in 10 or 30 square meter packs, so a bit much for a window box, and cost about £12 or £30.
For window boxes, have you considered copper tape?