Allotments 4 All

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.

Could it be Purple emperor with a bit of albino

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_irisThe 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.