Ok, I have read the (very recent) bug and re-read bits of it and am not sure that I am clear on what the solution is - this seems like it is an ongoing situation.
BTW, for clarity:
[root@server ~]# rpm -q httpd
httpd-2.2.3-83.el5.centos
From bug report -
"I suspect we are seeing a backup/restore issue here, resulting in missing 'apache' group."
"I then did yum update, followed by the signal-event ... after which the apache group was gone."
"I cannot see at which point the apache group was removed, although by time of the post-upgrade event it was gone."
"Experiencing same situation, but with a restore from sme8."
... does this mean I should file a bug report?
Basically, it looks as though I have a bigger problem than I had realised. Perhaps HF's method is the way to go, after all.