This turned out to be a bit of a weird one. I did buy another cable in the end but still the same problem. Doing things back to front (as I often seem to do) I then decided to try and copy the files to a floppy (using MessySID 2) rather than send the Setup over the null modem cable. It worked first time, so the cable was fine but the transferring of the setup files over the cable just didn't work. So advice to anyone else - if you have problems try the floppy method if you can before going for another cable!