I keep seeing a particular bird lately and I don't recognise it. Can anyone help?

I've only ever seen it from the car and in flight so I have no photo's unfortunately.

 - It's black and white
 - about the size of a magpie (possibly bigger, has broader wings)
 - Has very "square" short but wide wings in flight
 - Has a strange "swooping" flight pattern, tips it's wings backwards and forwards flashing the distinctive coloured underside
 - The tops of the wings are black
 - The underside of the wings are black and white, black on the tips with a wide flash of white
 - Distinctive flash of white on the underside of the neck.

At first I got excited thinking it was an osprey but I'm now pretty sure that's incorrect, it doesn't appear to be bird of prey too me. I put the weird flight pattern down to it being mobded by crows but it seems to always do it. Almost like a display. 

the first time I saw the bird it was in a field next to the sea. So I thought it was a sea bird, but now I've also seen one inland by my house. So I think it's a farmland bird. I've only ever seen one or two, never a flock. I live in North Wales. I've tried googling it but I don't seem to get any results...