Sacha wrote:

This is the unknown butterfly seen in one of the greenhouses a week
or so ago. I still think it must be an escapee. Sadly, it hasn't been
seen again. http://www.flickr.com/photos/93694401@N03/9562967292/

It's a Postman Butterfly:
http://en.wikipedia.org/wiki/Heliconius_melpomene

--
Chris