I don't know if Merlin Bird ID covers the UK, but I use it here and it is brilliant. They have just added a sound ID. I recorded 10 seconds in the nearby woods, and it identified 3 birds immediately by sound.
I also keep a bird book by the window. Darn little things don't keep still long enough for me sometimes! I will take a photo and then I can zoom in afterwards. Many of the little songbirds are very similar.
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