Whilst on the top I heard a strange 'pi-ou' call - a bit like Golden Plover.
Two pale grey Buzzards then drifted up the valley with the rear bird calling sporadically with the whistling call. Couldn't really get much idea of the wing angle but it did seem very flat or even downward, rather than a V. Also seemed quite long-winged compared with a Common Buzzard. They glided west - east with barely a wing-beat.
I'm pretty sure this was a pair of Honey Buzzards.