I would like to clarify something with the load order instructions in the readme.
Once I have un-rared, is the right pathway to have the Meshes folder go into the meshes folder in Oblivion data i.e.Oblivion\data\meshes\meshes and the same for the textures folder ?
Am trying a re-instal now with your mod as the only mod installed to see if it will work. Guna get this svcker going if it kills me....lol
No..

neither of the paths should have two mesh or texture folders. Just remember that. ^^
The paths are the same as it should be, I did make it more complicated than usual ( I apologize for that) but if you follow the install instructions on the "Description" at the link, it clearly shows how to do it, ie .... well I can't see the link now because I'm getting an IPS error. However, I'll explain in detail here.
First off, you can bypass all of this if you download the http://www.fileplanet.com/209164/200000/fileinfo/Oblivion:-Underdark:-The-Realm-of-Northdark-v0.59.1b
The instructions to install the archives manually are as the following. Please follow instructions to the "T".
1) Download all files first and place in temporary folders.
2) Place the contents of the archives Meshes1.rar and Meshes2.rar in Oblivion\Data\Meshes\
3) Place the contents of the archives Textures1.rar - Textures7.rar in Oblivion\Data\Textures\
3) All other archives contents (Sound.rar, Music.rar,
Underdark V_0.59.1b.7z) in Oblivion\Data\
To clarify more, It should NOT be Oblivion\Data\Meshes\Meshes\ or Oblivion\Data\Textures\Textures\