Robins mentioned, only copy Willow Warbler song at about this time so it's noticable when not included, could buy a recorder to prove things. Dad taught me quite a bit about wildlife and more than 60 years ago knew the songs of commoner warblers Chiff Chaff, Wood, and Willow Warbler.
If we did hear a W W it may have been a pal of this Swift:
Before dusk first week of December 2007, saw a Swift flying around the houses in Swanage for about 20 mins, then disappear into the gloom. Next morning got there early and stayed a while but didn't see it again. We haven't the slightest idea why decided to have such a long holiday.
Phoned the local reserve warden but wasn't sure if he believed it or not.
Chiff Chaffs have overwintered in Dorset over half a century in small numbers, but didn't see any during this colder winter.
Downsouthers