is wales tonight ITV? (as in london tonight and anglia tonight). If so it won't be on iplayer because that's BBC. I don't think they archive old broadcasts. You could always check that out on the itv wales site.
Just a thought, it might work if the links still exist. Join wales tonight on facebook, get their home page and scroll back, and you may come across a "watch the programme" link for the day you want. They do with london tonight but I've never tried watching it from a facebook link.