Peacock caterpillar - is it late?

Yesterday I saw my first black caterpillar of the year, the one that feeds on nettle leaves, and eventually becomes the Peacock. It's the same butterfly that sunbathes in our windowsills and can be seen flapping around in the middle of winter.

When I saw the caterpillars I thought they were a bit late in the season and checked back; sure enough, in 2011 I had seen them on the 7th of May. Summer has been poor, but surely not that bad?

I've had a look at the Butterfly Conservation website which says that in a good year they may have a second brood. Presumably that's what has happened and I have been imagining all this cold, damp weather.

'This butterfly is generally single-brooded. However, in good years, a small second brood may appear. Adults may be seen at any time of the year, with warm weather waking them from hibernation. The majority emerge from hibernation at the end of March and beginning of April. These mate and ultimately give rise to the next generation that emerges at the end of July.'