Yes, if the OP shared a lot more info than he said, then I am wrong. Even then this data wasn't released or made public, it was used inappropriately.
"Hello we have your number on file as the emergency contact for Mr Bloggs, are we speaking to the correct person?"
But I have no idea. I've not seen the form, I did ask what the OP had actually agreed to and I formed the impression that it was just a phone number. I get the feeling that the OP didn't even keep a copy of the form, being a casual job I guess he didn't really think about what he was completing or consenting to.
Let's assume that the employer is found guilty of a breach of the DPA. This would need to be a private prosecution because Ofcom won't act on a single instance. What material harm has been caused? I'd already said that this was probably enough to get him out of his contract if that's what he wanted.