The butterfly that hatched last week is back. We let it go because Common Brimstones will often fly early in the year. He flew away and that's when I started to think that this wasn't such a brilliant idea - he had just hatched after all and he had no reserves to get him through cold spells.
For some reason, the butterfly returned to our balcony and we took him inside again. He's now residing in an improvised cage with room enough to fly a bit and two primroses, the only flowering plants to be bought at the moment. He also gets honey water and a couple of daisies and he seems to do well. We'll let him fly again in two months or so, depending on the weather.