What is your understanding ...

[Polchraine]

or what is legal.

Some land is owned or has permission for shooting on. The quarry is identified and a safe shot can be made from elsewhere on the land however the projectile - bullet, shotgun pellets or air rifle pellet, must travel a shot distance over some other land - not public highway, about 1 metre wide. Provided the projectiles do not land in that small strip and no one is endangered or damage caused, is it legal?

An example would be where the land is "L-shaped" and you shoot across the corner.

I believe it maybe/is illegal - but others suggest it is acceptable.

[Blackstuff]

You need permission to shoot from the other land owner too to be fully legal. Its definitely mentioned either in the guidance or legislation

[Graham M]

The law states that any shot/projectile must not leave your land, so technically shooting across the corner of some else's land would constitute an offence.
It would be a bugger to prove though.
G.M.