It is a butterfly or is a moth.
A not infrequent question when you come across one of the day flying moths such as this Latticed Heath. It sits like a small butterfly, it's about the same size as a Chalkhill Blue butterfly and it shares the same habitat. It is a moth in the family Geometridae, and taxonomically it is more closely related to the Butterfly families than it is to many moths.
My conservatory, where I empty my moth traps and try (not always successfully to identify what I've caught, it of course a great place to live if you are a spider. If it thought moths were difficult, then spiders are the next step up. I've looked for this chap but can't find his ID. (it's Araneus diadematus, see Rob's note below). I lives in the corner of my mothing room and has a fairly small web.
It is successful, and although I try and ensure the safe departure of moths that I bring in with the traps, it does sometimes catch one. It wastes no time in dashing out and imparting a lethal bite on the hapless insect. It also does a good job on the mosquitoes that I invariable bring in with me, as is whitnessed in this larder.