JohnfromUK... a couple things to consider on the incoming bird. If it's going to be truly overhead, the longer you wait the easier the shot is! This assumes that you can indeed swing the gun to a high elevation, but if you wait until it's near overhead the gun has a maximum amount of built in lead. They're also closer at that point, and they have a bigger profile.

Shotguns have built in vertical lead at high elevations. Look at the angle made by the bore and the rib. This is to compensate for drop caused by gravity.

When shooting overhead, that need disappears.

The real trick is to swing from behind, overtake, and as soon as the bird disappears from view (covered with muzzle), shoot and keep swinging.

If you see the bird when you pull the trigger, you've missed behind. If the barrels block the view you're in front and you have a chance.

It's my favorite target.

"The price of good shotgunnery is constant practice" - Fred Kimble