You can probably find a template off the web, which you might get for free, or you might have to pay a few pounds for.
As a general principle, that would do the job. Of course lawyers will say they need to be employed and, sure, you limit your risk of foul-up by employing one.
Much depends how much is at stake.
If they run off with your idea, with or without an agreement, you'd have to figure out how much money they made from it before deciding to sue them - no point suing an organisation that has no money.
Which is kind of why the legal fuss about pinching the Da Vinci Code book idea came to the courts - the writer had made a shed-load from it and the other party felt they wanted a slice.