could it be oil seed rape?

No, leaves are wrong for that.

sugar beet is a possible and I know pheasants are often found on them. many years ago we had friends who used to shoot pheasants on the local farm land in Lincolnshire.
even old fashioned mangle[worzels] as a sheep fodder is a possibility.

Not either of those as there is no sign of any bulbous base to the shoots. Sugar beet is not a Brassica and these really do smell cabbagy.
One of these
https://edwintucker.co.uk/seeds/farm/game-cover/brassicas-2/

Maybe one of the many mustard greens?

Well I always thought beets were a brassica, never looked at the flowers so not really noticed. learn something every day.

Beta maritima is related to Amaranthus.

now I've looked at the flowers I can see they are related.

Could it be kale?

Owdboggy, I've been reading up on pheasant husbandry, and it could be one or more of a wide variety of "greens". Since they smell like Brassicas, they're probably a member of the mustard family, but the only way to tell which is to actually see the shape and color of the leaves.

Bit of a long walk up to the fields carrying my camera so I think I will pass on that.
May ask the Gamekeeper if we ever see him.