Well I'm loathe to answer my own question but I have resolved the situation. 
Since the problem was down to the changeset not being able to use the right test with the right class I decided to find out how to delete the useless test class.

Thanks to the excellent information from @sfdcfox in the answer here:
http://salesforce.stackexchange.com/questions/46612/how-to-delete-a-test-class-without-using-eclipse

I was able to delete the bad test class from the production org, then push the changeset with the new class and new test without any trouble.