Bit late, but there are Kestrels in the area, so it was probably one of them. They did breed a few years ago in that electrical building on top of the bridge to the velodrome. I had 3 juveniles on the east marsh late summer, but don't know where the recent nest is.
It could've been a Sparrowhawk, they bred this summer in Wick Wood. But I've had more Kestrel sightings near the hockey centre.