I don't know the extent of how similar or not the UK's copyright system is to the US, but I assume frob is from the US like myself.
I feel like Stu miss a fundamental concept in what copyright actually protects; at least here in the US. Copyright does not protect ideas itself. Copyright protects the expression of an idea put into a tangible medium.
Stu is correct that people can copyright their written story, audio/visual artwork, and whatnot IP from direct or substantial copying, duplication, and distribution. However, copyright does not protect the ideas used within the work no matter how original it is. If Disney or Nintendo can't stop knock-offs of their IP what makes individuals think they can. There is a reason why there is 50+ clones of any popular game.
A storytelling “mechanic” is The Hero's Journey which is not copyrightable since it's just an idea/procedure. The a unique story made using The Hero's Journey is copyrightable.
A fairy is an idea of a mystical creature which is not copyrightable but a fairy uniquely expressed in drawing is. People may feel they have created a 100% original work, but the truth is that they really have not. I'm not trying to be mean if I come off that way.
When it comes to game mechanics, it's not copyrightable. However, if a game mechanic uses some novel and complicated algorithm there is a potential to patent it. But don't be a patent troll like Sega by getting a patent on a arrow indicator in Crazy Taxi, or crowd avoidance that is just three levels of radius check over on a point.
When it comes to protecting ideas within written documents copyright can help limit/restrict the distribution. However that does not help if they just remember what was in the document; no copying required. To truly protect your ideas is to have people/organizations sign a non disclosure agreement.