Hmm, something curious has gone on.
Whilst looking at one of our servers that wasn't syncing to ntp0, but to pool.ntp.org it had inserted a leap second last night at 23:59:60. Ntp0 however didn't. When I fixed the config on the server to sync to both pool.ntp.org (this time a different server) and ntp0.zen.co.uk it showed both as being 1 second offset from its time.
It would look like our server had seen a spurious leap second from 20six.co.uk.
On mentioning this to our Principal Engineer who also keeps an ear to the ground on time issues, he located the IERS document :
IERS Bulletin C
Which states that there would be no positive leap second at the end of June 2008 - so the leap second that we'd seen was indeed spurious. That is rather concerning as how far round has that error propgated ?