Submissions do not consist of code, merely predictions (at least for the competitions I'm familiar with). I don't think there is anyway they could compel you to give up your code, but I'm no lawyer.
The winning entry has to be a general algorithm that can be implemented by the RTA. That means that it takes a timestamp as an input and can generate predictions for the next 15 mins, 30 mins, 45 mins, 60 mins etc. Results must also be replicated by the RTA before any prize money is awarded.
So it sounds like they are expecting your code.