Tomato Seedlings Dying

I've grown tomatoes from seed for about 10 years now but this year, the seedlings have started dying :-(

Aisla Craig

It look a bit like slug damage but I've never know the local slugs eat tomato plants. I've not noticed any slime either. When not posing for the camera, the plants are in a mini-greenhouse so cold weather damage is unlikely. Especially since some other more sensitive seedling such as Physalis are unaffected. Hessayon  suggests woodlice. Might be the answer because take a look at this monster grafted plant and in particular the area in the red box. The damage on this lower branch looks like something ate it. So it's probably not a cultural problem.

Answers, as they say, on a postcard.