sounds like reed bunting, but could it be a willow/marsh tit?
There are some similarities between those birds. I get Marsh Tits in my garden, they’re not unlike a Coal Tit, but with a generally olive green body and wings and a black head with a black nape – Coal Tits have a white nape. Reed Buntings are more sparrow-like in their markings, brown and black streaks and grey underparts, apart from the black head on the male, with the white dividing band around the neck. I don’t think they could be mixed up.
What can be mixed up might be a black cap and a marsh tit – I get both, there’s been a black cap pair coming into my garden right through the winter, and I had a marsh tit the other day, and there are similarities;
top to bottom – reed bunting/marsh tit/black cap