Thanks all, Dave's website has virtually convinced me what I saw was a merlin, but its the sparrowhawk images ruling out sparrowhawk rather than anything else
The wings were narrower and more pointed than sparrowhawk, and with no suggestion of the 'fingers' which are apparent in almost all of the spawk pictures.
As for the hovering, it was a 'proper' hover, using mad flappy wings, (rather than just using the wind) it just wasn't sustained as a kestrel would do as the birds it was hunting were at the same height as it, i.e. it wasn't watching/looking for prey on the ground. It was basically holding its position as the starlings swirled round it.
I agree with Dave, officially 'unidentified' but in my opinion, probably a merlin.