Could be a nightingale.
Referencing this site it does sound possible to be a Nightingale. (inc' audio extract).
http://www.garden-birds.co.uk/birds/nightingale.htmAlthough it mentions the birds' movements (UK) as follows:
"They usually arrive in April and leave around August to September. They winter in central tropical Africa."
then goes on to mention:
"Additionally, the Nightingale may be struggling on its wintering grounds (Africa)."
Well African Winter is June, July and August, their temperatures (16c) are roughly double what is recorded here for me (7c).
Based on that information if they are Nightingales here in "our" Winter they must have decided to skip the flight this year, perhaps adapted; probably wearing thermal long-johns knitted from weave grass.
Thanks for the info Paul.