Hi Richie,
Once you have put down your game idea on paper - or more likely in this case created the code for the game, you will own the copyright in it. There are various things that you can do to strengthen your case should it ever be necessary to prove your version of events in court, such as lodging a signed and dated copy of whatever physical medium you have recorded your game idea on (paper, CD, DVD etc) with your bank, a solicitor or one of the copyright registry services such as
UK Copyright Service. (Note that this is commercial business and that there others which offer the same sort of service.)
But really you best bet is to make sure that anyone with whom you have to share your idea is bound by a non-disclosure agreement. You can get a specimen NDA
here. Once someone signs an NDA with you they owe you a legal duty not to breach your confidence, and if they do, they could face penalties in court. By making them sign a NDA you are drawing this fact to their attention in a more focussed way that just relying on copyright alone.
I think you are very wise to take the greatest care to avoid your idea being pinched by someone else, given the competitive nature of the games industry. Obviously you strengthen your position by not revealing more than is strictly necessary to each potential collaborator. For instance the illustrator who creates your sprites does not need to know much about how the actual game will be played, and similarly anyone who writes music or sound effects doesn't need to know what the characters look like, and so on.
You also need to establish from an early stage who will own what rights to the various elements which you outsource. For instance unless it is agreed otherwise beforehand, the illustrator will own the copyright in any artwork he produces and could in theory sell this to another game developer unless you draw up a contract with him which assigns all copyright to you in exchange for whatever fee you agree. The same will apply to any original music you commission. You can obtain suitable agreements for this purpose from the Own-it site mentioned above.
You should find some additional information here:
Business Link
Good luck with your project.