In reply to CurlyStevo: I'm surprised that you would be able to see anything moving at that speed what with your age related sight deterioration mate
If it's moving at that sort of speed it's likely to be a peregrine - they catch their prey (usually other birds) by going into a steep dive where they literally knock the other bird out of the sky. I've seen loads of them at Swanage. Also they like high vantage points for hunting/nesting.
I would think at Stanage it's likely to be a kestrel - the males are a very clear grey and brown and are not massive - they also hold/hover against the breeze.
Although near to moorland it could even be a harrier ?