ID of a Highland bird please

I saw this little bird by a Scottish loch last week. It was tiny - about the size of a wren. There were thousands of them, flitting about in silver birch regrowth. I hear no 'song' but continuous 'cheaping'. I suspect it is some kind of warbler, but there are many kinds. Any ideas? Thanks

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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
I'm really no expert whatsoever .... but perhaps a snow bunting?
Bee x
A single bee creates just one twelfth of a teaspoon of honey in her lifetime
When you don't even know who's in the team
S.Yorkshire/Derbyshire border