@mitu I've got it working now, I had a typo in my file, I had missed out the / before dev/input/. Schoolboy error but sorted now. Many thanks for the pointers and your help. Working between my PC and the cabinet is quite tricky. I'll need to get SSH sorted out so I can do the edits from the PC direct.
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first