A tephritid fly, probably Tephritis hyoscyami (Diptera: Tephritidae) on a thistle (probably a Carduus sp). There were quite a few of these of on a stand of thistles which were about to come into flower. At first I thought that, with the two dark blocks on the wing, they might be Tephritis bardanae, but I had previously only seen the latter on burdock (Arctium sp) so I witheld judgment until I got home and had a chance to look up the wing patterns. Then I discovered that they were most likely T. hyoscyami.
Photos taken in the field below Chazey Wood, near Caversham, UK, on 2012-05-22.