I signed up just so I could reply to your question, Jo. Hopefully you regesterd your art with the copyright office. Never give a company all rights to your artwork!!!! You can re-license the art to other companies over and over. If that's what they want, pass it up. Also don't get a flat fee, got royalties. More lucrative. They need to specify what the art is going on, for how long, the royalty fees, and how it will be paid. Ideally you should have a copyright lawyer look over the contract. The company will take you serious and you can get better options. Look up blogs from Tara Reed, Maria Brophy, and Joan Beiriger to start.
Read this link with simple basic info on it: http://artmarketing.com/licensingart101/lice.html