Help me identify this weird bird of prey?

Hi, just hoping that someone could help me identify what the bird of prey I saw this morning was. I'm familier with kestrels, sparrowhawks and buzzards as we get many of them, and it was definatly none of these, and I'v not found a good match for it in my bird books.

It was considerabley larger than a sparrowhawk, the tips of its wings were quite rounded, but not stubby, and had a long straight tail, longer than a kestrels proportionally, and it didn't fan it out. It was backlight but it 'might' have been dark brown? But the thing I thought odd was that the trailing edge of its wings were bright white from the body until about halfway along. It was gliding so I couldnt see it's wings moving, but it shifted them back in a sort of semi-stoop as it passed over the house.

I live in arable farmland with some woodland and large lakes a few miles away.

I'm at a loss as to what it might be, my bird books are old and not very user friendly, but it still didn't match any in there, mainly because of the white on the wings, I'm wondering if it was a falconers non native bird that escaped?

Any thoughts as to what it could be would be greatly appreciated!