Thanks very much for the quick response, Grinder. Although your
suggestions didn't directly solve my problem, they did give me the
confidence to keep looking before buying a new graphics card!

The solution that worked for me was to install an ATI "Omega
Driver" (not supplied by ATI) from here: http://www.omegadrivers.net/ati.php

After that, the resolution I wanted was available and works perfectly!

Hope this is useful to someone else one day when Googling for answers!

Cheers,

Andy.