Yep Luke that was my first impression as well, particularly the barring and shape. The square tail has me stumped. Could just be the angle of the photo perhaps?
Think you guys might be right. Looking at field guide, it mentions the Peregrine has barring on breast and that the Hobby has a square-cut tail proportionately longer and narrower than Peregrine, which I think this photo also shows. My initial thoughts on the sighting were Peregrine on size, but perhaps this was a female Hobby? Happy for it to be changed.
I agree with Matthew that this is most likely a female Hobby. The key features for me are: very pointy/sharp wing-tips with a slender curved trailing edge, tail longer than a Peregrine, lack of yellow on legs and rufous colour of plumage. The colour is possibly similar to a juvenile Peregrine but the wing/body shape for me is what differs here. After doing some more research, the square tail is common to both when the tail is furled. So much to learn!!! ;)
It was a very thrilling sighting as it came over twice, the first time as shown in the photos and then about 10 seconds later it came back in an incredibly fast stoop.