Having tried everything else I could think of I decided to remove the Kix RPC service and delete all Kixtart files. I then rebooted all the servers and reinstalled KXRPC and replaced all the other files (having once again downloaded a fresh copy).

Guess what? The ingroup function is now working as it should, at least with a test script. I can't explain why but I'm sure glad the problem is fixed.

Thanks again to all of you who took the time and trouble to help.