I'm puzzled by a butterfly I saw on the allotment today. It never landed that I saw, so no pics. It was big, a strong flyer, and looked very like a White Admiral, except that ther was a large irridescent purple spot on the hindwing, and the white markings were confined to the forewing.
Could it have been a swallowtail? It's a big string flier, and it's similar to what you describe, though it's got a bit lost if you saw one in Brum as it's supposed to be restricted to the Norfolk Broads.
(http://www.ukbutterflies.co.uk/phpBB/gallery/images/upload/602987a0ca9895d06564c6a7bb15694c.jpg)
Could it be Purple emperor with a bit of albino
(http://i1208.photobucket.com/albums/cc377/hippydavec/508b37c22e6975e03268977a45b16de3_zpse11ab50d.jpg)
Definitely not a swallowtail, I had a really good look at it. Purple emperor had crossed my mind. I need a book with all the variants, but unfortunately the Central Library is closed due to a move, and won't be re-opening in the new building till September.
http://en.wikipedia.org/wiki/Apatura_iris (http://en.wikipedia.org/wiki/Apatura_iris)The picture on this site seems to show a purple marking on the smaller wing.
It was more of a purple spot, with no purple on the forewing and no white on the hindwing. I just wish I'd been able to get a pic. I'm pretty sure it was a colour variety, which means it was probably something rare, maybe even rarer than the pale clouded yellows I saw in Cornwall years ago. But I've no proper record, and the central library is closed till September while it moves to a new building. Aaarghhh!
use this site it where i identify most of my species from.
http://www.ukbutterflies.co.uk/index.php
go to the species tab and browse from there or click Whats that Butterfly in the top right
Comma. I'm as puzzled as ever; I think the only hope is likely to be a specialist book on UK species.