I'm also going to stick my neck out here regarding your remaining "wanderer". This looks more like a Red Admiral caterpillar to me. Does it have a single white line down its side, rather than a double line? If this is a Red Admiral larva, then it will tend to make home in a curled up leaf (which matches your description of it "hiding" among the leaves), and it will also form a tent out of leaves within which it will pupate.
That is so interesting because I was confused about the one line down the side and it is bigger than the others too. It kept curling the leaves around it before.
Do they eat nettles too? There is not much left but we are finding it hard to get some more and they are on the roof so an scared to open it incase we damage them.
Will we have to go on another nettle hunt?
I'm afraid so. Yes - they eat nettles too. Perhaps you can remove the remaining larva and put it in its own container. Or try and remove the lid without disturbing the pupating Small Torts.