While I thank you all for your prompt replies it might have been useful if some of you had actually downloaded the test module that I linked to in my original post as some of your questions would have been answered. So here are some images to hopefully answer those questions.
This shows the whole of the testing module in it's entirety. As there is only the single waypoint and the single generic trigger I left their tags untouched.
This shows what little I have put into the module.
This shows the single area that is the area in the module.
This shows that I am using the standard palette and the undead listed under the other category.
This shows what I get when I do an edit copy on the Mohrg
This last shows what I get when I click on the advanced tab. As can be seen the resref is as I have described it and I can place this Mohrg into an area normally in the toolset.
There is nothing in my standard override directory and nwn.ini points to the standard override folder. If anyone's interested I created an xls spreadsheet that has the following information in it - the name, tag, resref and CR of all the undead monsters in my standard palette. I made it for myself but in case there are other undead with dubious resrefs that someone would care to point out to me (I have had the same problem with the Vampire Warrior), it can be downloaded from here (this is so tiny I didn't compress it).
I don't know if it makes a difference but I have NwN diamond (disk version) patched to 1.69 and all the premium modules.
TR