In reply to Michael Hood:
I am not a bird expert, don't want to be the boy that cried sparrow! They definitely had a distinct raptor profile and pale grey/white colouring (no red like a kestrel). There were loads of jackdaws nesting at millstone, I know what they look like, definitely wasn't them.
I say 'crack'....it was in amongst the broken boulders/ledges/cracks on the left hand wall of the pool. It was the pale flash of wings as it dove in there that caught my eye, and then I saw the other one alight at the top of the crag.
Thinking about it, and what other posters have said, it's likely what I saw was peregrine pursuing a jackdaw which fled to it's nest.
If others can keep an eye out round there anyway.....
Post edited at 07:52