Not sure how many Chiffchaffs winter that far north and I'd have thought it was a bit early for a spring migrant, though not impossible.
If it was really behaving like a flycatcher, i.e. leaving a perch to go on an aerial sortie chasing flies and then returning to the same or another perch, and repeatedly, then I'd be inclined to agree with Mike, despite Chaffinches obviously not being smaller than Chaffinches! If it was just hovering close to the tree and picking off insects from the leaves then some of the smaller birds can be more possible - Goldcrest would be a particularly likely candidate.
Regarding size, I always beat this drum, but most birders, even experienced ones, fail to realise how easy it is to get the wrong impression of size. Our brain tells us a bird is the size of whatever it is we've first identified the bird as. Usually our brain is right as we've identified the bird correctly, or at least as something that's about the right size, and this reinforces our confidence that we are actually judging the size correctly. In reality we're not, we've just identified the bird right for other reasons, perhaps subconsciously, and our brain is deducing the size. So, if you think Flycatchers are smaller than Chaffinches (as presumably you do from your post) and you catch a glimpse of a bird that you first think might be a Flycatcher because of how it's behaving, you will judge its size to be smaller than a Chaffinch. Even if you had gone on to see that it was a Chaffinch (I'm assuming here that it was a Chaffinch) then your brain would have thought it was a small Chaffinch.
I was once watching the sea from a clifftop when a dark-bellied wader approached over the sea. I can usually identify waders passing by at a distance by jizz pretty accurately but for some reason my initial impression of this one was wrong. My instant identification was Golden Plover but as it approached it immediately became clear that it was in fact a Dunlin. By this time though I was convinced it was Golden Plover sized and had some difficulty believing it really was a Dunlin, despite the now indisputable features proving it was one. Even when it eventually came right close to me and I could see all the detail I wanted, it still seemed like an ENORMOUS Dunlin. I was with a friend who hadn't made the first mistake and to him it just looked like a perfectly normal-sized Dunlin, which I'm sure it was.
Anyway, back to the bird - there's nothing else I know of that behaves like a Flycatcher nearly as effectively as Chaffinch. If I had a pound for every time I'd seen someone mistake a Chaffinch for a Flycatcher I'd be, well, perhaps not a rich man, but considerably better off! I have seen other species behave like this but rarely so convincingly and not usually for more than one or two sorties.
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