Are you growing them up there in northumberland ?
I tend to think that pigs are physiologically pretty much the same as us. They do not have the same seity mechanism I guess ,so they will eat all you give them.
I guess if you have concerns then try to store as many of the fruits as you can and limit their daily feed. We do this with apples but fruits like plums are difficult to store so they get all we can gather.
Maybe check they are not getting the trots !! ??