Multi-key on UTF-8

I was previously using the ISO-8859-1 encoding, and have changed to UTF-8 by setting the RC_LANG variable in /etc/sysconfig to cy_GB.utf8 (which I built using localedef). With 8859 I was able to use the Shift+right-hand Ctrl keys to allow entry of ô, etc. However, after changing to UTF-8, this no longer works. Do I need to do something else to enable this again? Or does it not work with UTF-8? /usr/X11/lib/X11/locale/en_US.UTF-8 refers to a deadkey, but it's not clear which one it is. I'd be grateful for any advice. Thanks Kevin