Unidentified bird call

I Live in rural Midlothian, Scotland. We're surrounded by woods and fields. This Summer I've heard an unusual Bird call a few times but haven't spotted the bird yet.

It's a very distinctive 'Wow Wow Wow Wow Wow Wow'  call, heard only in the daytime and I assume the bird is sitting in the tree when making the call. It'll repeat a few times over a number of minutes. Going by the depth of the call I'm guessing it'd be about Wood Pigeon size.

Any help much appreciated! Let me know if you have any questions!