Our daughter lifted a strawberry and caught one in the act. Apparently some varieties of woodlice can only eat fungus, moulds and humus, but others eat soft plant tissues, especially seedlings.
I overcame my laziness and googled, finding http://www.small-farm-permaculture-and-sustainable-living.com/garde...
I have set a trap tonight - a dampened newspaper pouch baited with grated cheese and potato slices. In theory they should feast and decide it's a nice place to stay for the day. (The other recommended bait was strawberry!)