Matthew, you must still be working in 6.1 because in 7.0.2 Field Properties doesn't have an Aliases option. I guess that was their solution for that error.
Thanks for the reply. As you say that is one way to fix the problem. A calculated field it will have to be in that case.
FYI the option was still there up to 7.0.1 but it's good to know that there's an incremental upgrade out.