What animal is making this noise?

I have to admit the thrill of hearing our UK wildlife on my walks with the dogs; the cuckoo in the spring; the woodpeckers in the summer as well as our usual bird song throughout the year, but out on our walk recently I kept hearing this noise.  I have never heard it before; it was coming from high up in the trees, so I am assuming its a bird of some kind????

If you have any idea please leave a message in the comments box below,I would love to know!

Oh and ignore the dogs panting :-)