You do get Spot Fly's there as well. Are you sure it wasn't a Spot Fly?
No, far from it, but I haven't seen anyone else reporting Spotted Flycatchers as having arrived yet and I'm still not confident on calling 'new' species ahead of the vastly more experienced folks who also post here. Were there any Spotted Flycatcher reports yesterday?
Looking at photos of both species online, I was viewing the bird from behind and so would have missed any steaking on the breast, so I'm going purely on the similarity in size and appearance from that angle. I did see at least one Garden Warbler singing from a similarly prominent position yesterday, but the fly-catching behaviour loooked more like what I've seen from Spotted Flycatchers in the past.