EBarnacle,

I think you are in the UK, so I wouldn't have any kind of clue.    Here in the SE USA, a bird spending the night in such a place would most likely be a wren.

I positively identified a Yellow-Rumped Warbler this afternoon on the suet feeder. I think there have been a few of them around all winter, but this is the first time one was close enough for long enough for me to make a certain ID.