Birds nest - can anyone help me identify it

Hi, can you give an idea how big the nest was?

I found it quite difficult to gauge size. Much bigger than a blackbird nest. About 1/4 to 1/3 the size of an osprey nest. I had originally thought crow/rook but each time I've seen their nests they tend to have twigs pointing out in all directions. Also these are much bigger than twigs - they look as if they would be about thumb sized. From what I've read rooks tend to have other nests around - and this is a solitary nest. The following are the bird species that I have seen (but there may be others I haven't): rooks & crows, tawny owl, buzzards, red kites and another smaller raptor that I've seen on the wing but it was past me before I could work out what it was.