Which specific model of monitor do you have?

Edit:

Have you set Systems->Preferences->Hardware->Screen Resolution? This one is user(non root) selectable.

I have the Samsung SyncMaster 2493HM.

Aaah! I completely overlooked that. Not something that I have used for ages, hence slipped under the slightly ineffective radar!
After booting, and resetting the screen as usual, I checked progress on LinuxQuestions and saw your posting.
When I checked the setting it was at 1920x1200, but the check box "make this the default" was not set, so now it is set as the default, and the resolution is now 1920x1200 on login after a logout. I am assuming at this stage, that is effectively the same as a reboot.

Thank you very much for your persisting with your help and advice.
It is much appreciated. I can always count on good support from the members on this site.

Cheers,
Terry