After reading the blogs of other ringers who were fortunate enough to also trap and ring red-flanked bluetails last week, I noticed that most of them had sexed their birds. Svensson (1992) says that juvenile birds can not clearly be sexed unless there is blue in the lesser and medium coverts, thus indicating a male. However Mark Newsome kindly forwarded me a copy of British Birds 102, September 2009 which featured an article on ageing and sexing of Asian chats and included red-flanked bluetail. Males clearly have a much brighter blue rump and tail feathers over females. The females also show a greyish cast on the tail which is lacking on males. On review of photos of the Whitburn 2011 bird, I have come to the conclusion that this was a juvenile male. The photos below show how bright the rump is on this bird.
© Chris Bell
© Adrian George
After reviewing Paul Hindess video on youtube of the 2009 Whitburn bluetail, I believe that this was also a male.