Yeah, we commonly call them woodlice in the UK but I've just looked up Sow Bugs and they are the same thing. There is a common science experiment in high school where you put them in a tray with dry, damp and wet compartments and watch them all make their way out of the wet and the dry areas and in to the damp area. Didn't know that about crayfish being related to them but can see the similarity.
We have masses and masses of them in our compost heap but we leave them be there as they help break down the stuff we put in. That's not close to the greenhouse though but I think the things are everywhere.
Lastnight when I watered some of my orchids which had been on the floor I soaked them right over the top of the medium and lots of things including mini slugs and snails, a centipead on one, and lots of tiny woodlice on everything came crawling to the surface where I fished out everything I could see