Neil, my thoughts initially were also juv female sparrow hawk. You must have posted thinking it might be goshawk. If the pigeon is a woodie and not stock dove or feral pigeon the size indicates a large hawk. Your friend's pathway has helpful even bricks that can be measured and the bird is standing at a convenient angle. You, like me, may have had to write out Pythagoras theorem 100 times for some minor offence and so could calculate the size of this bird. In conclusion I think there is a significant chance this could be a goshawk.
Happy Xmas All
David