RSPB conservation director Mark Avery has blogged about a nesting pair of Purple herons at their Dungeness nature reserve. There are a couple of good photos too. I think if you saw one in the wild and weren't aware of such things as Purple herons (I wasn't before today) you'd probably convince yourself it was a 'normal' Grey heron demonstrating quite a severe variation from the traditional plumage, or perhaps imagine that it must be the light.