Jon, I think I would have to call this a Kestrel; it was my first impression when I saw your photo though when I thought about it I realised that the features of it were not clear cut ( may I blame the shot through the screen?! ) To me the back looks reddish- brown, the head looks greyish and the moustachial stripe looks to be heavier than that of Merlin. The features of the underside of the tail are in shadow and I would like to think that it is not as black as it appears!
Many thanks, Tom. Yes, I rather suspect it is a kestrel, so my hunt for a merlin must continue. I just wish I'd got a better shot of it - yes, this one was taken through the windscreen.