Oh dear, I feel I should hang my head in shame ! I blame it all on lack of coffee... Many thanks to all that took the trouble to reply to what turned out to be my own self-inflicted problem.... It turns out that I had upgraded one of the servers to PHP5 to do some testing and had left it that way rather than reverting to the stock Centos PHP4. That, coupled with a rather outdated install of Horde was causing the problem. Back on PHP4 now and all is ok. Thanks again to all, Andy.