True, the vnc server needs to be started after the rcxdm restart suggested by YAST but the Allow Remote Administration setting in YAST remains as OFF i.e. the radio button hasn't chaged from the default to the new value.
It looks to me as if the GUI fails to write a new setting to a configurtaion file somewhere behind the scenes and if this is the case I should be able to fix this by editing the relevant file manually. Trouble is I don't know how to do this.
The problem isn't confined to a single machine, by the way. I have tried this on three different hardware setups with the same (non) result.