I noticed that although I'd fetched the latest update to ntp from the mirror it wasn't being installed.
It seems that the version numbers have got out of step. According to the changelog the update that was released in August should have had the version number 4.2.0.a.20040617-8.el4_7.2, whereas in CentOS it's actually 4.2.0.a.20040617-8.el4_8.2.
The changelog says the latest update is 4.2.0.a.20040617-8.el4_8.1, which is what CentOS has, but that's a lower version than the August update.
Ron