The previous week I had seen another one and had left it outside but, when I checked back a couple of days later, it had disappeared (wasps). I didn't want that to happen again so this one was brought inside and put into the caterpillar castle.
After a day inside it made its button to hang from and then formed into a J.
24 hours after that, it formed its chrysalis.
Now we wait.
(I'm using phone photos here, so apologies about the quality)