String Custom Field Not Working Properly?

[Hawker]

I have followed the instructions to make String Custom fields, to the letter. This is to make two options to a single Product, appear at the 'select for purchase' stage.

Defined radio buttons appear correctly when customer selects product. At the cart stage however, the two options both appear listed, without any radio buttons. Surely at that stage of proceedings, the customer should only see in their cart, the option they have selected at the previous stage?

Please can anyone tell me what I am doing wrong?

Thank you.