Ah damn, some other techie decided to reboot the server to fix the problem (!) so I can't follow it up (until it maybe happens again!).
Out of interest though, I tried that command but it doesn't look like this version of linux supports it:
[root@bml root]# lsof | grep "pts/20"
bash: lsof: command not found
[root@bml root]# man lsof
No manual entry for lsof
Thanks for your assistance anyway