On behalf of Dave who sent in an email:-
Seen on a nut-feeder in my Cheltenham garden this morning, a bird I have never seen before. Similar to a warbler, sparrow or chiffchaff, but with a very noticeable slim mid-brown cap. Soft brown uppers and light underneath. Dark eyes. Brown legs.
I couldn't find it on your identifier, but on the RSPB's it seemed nearest to their image of a nightingale.
Any thoughts on what it may be?