"Quark!" bird

My husband wants to know what kind of bird he saw in Cumbria about 6 weeks ago:

It went "Quark" while at the same time tipping sideways (one wing to the sky, the other to the ground) as it flew along.  He says it kept doing this very distinctive behaviour as it flew.  The call was always accompanied by the movement of the whole body.  It was large, but he could not detect the colour because it was outlined against the sky.

Any ideas, please?