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: How to delete a Test Class WITHOUT using eclipseHow 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.