Been trying to find more info on this. The only thing I've found is on potential SFC sighting nr Bristol (I think it was). People on RSPB suggesting possible Dunnock, Stonechat(female) or a juvenile. Not convinced by any of those possibilities having looked at photo's.
*edit* Just looked at Chiffchaffs. Do they get streaked on their chests? Having said 'streaked' makes me wonder about it being a 'Spotted' Fly Catcher!? Its a long time since I saw one in the flesh(feather0 so to speak!