First of all, can you prove that you wrote the melody and lyrics? Do you have any email or written correspondence to back up your claim?
If you don't, you may be vulnerable on this point.
But the first thing you should do is to contact the producers, and politely ask them how you're going to be credited, and what the royalty split is going to be for your collaborative song. It's possible that they know as little as you.
If they're not intent on ripping you off, it should be as simple as registering with PRS and staking a claim to your share of the song.
If they won't play ball, the next step is to contact the record label. Start politely by asking them how you're going to be credited and paid for your song. If they don't seem interested in accommodating you, let them know you can prove your authorship, and that you know your rights, and are prepared to take legal action to resolve the matter.
If you're still getting nowhere, you'll need to get a bit heavy handed. You're probably going to need legal representation. The most cost-effective way I can think of getting the right kind of representation is to join the MU (Musicians Union) who offer free legal advice to members.
In fact, they'll be able to help you much more than I can.
Good luck, and let us know how you get on.