Enrico, you're a star! I'm used to using su on AIX where the command prompt and home directory both change as soon as you switch user's -
root@p25n10e0:/ # su - ops0052
I hadn't realised that this was not the case in USS. Using the ID command confirms that I have in fact switched -
OPS0052@PROD:/u/ops0052>£su -s ops0047
Previously the error was only occuring after I had issued the command the second time, i.e.; I had switched users and did not have authority to switch again from there. I should have twigged that the first command, whilst apparently not doing anything, was in fact working as expected.
Thanks ever so much for your help.