I downloaded a tileset which included a wall niche. But it was only for a room (with a higher ceiling and only one side) and I wanted one for a hallway, so knowing nothing about nifskope, I copied some blocks from the first mesh and added them to the second, the existing hallway mesh. After some experimenting, I seemed to get it to work by zeroing the vertices for one side of the 2-sisded hallway mesh. That's the side that the imported blocks cover.

The result is that the mesh displays the way I want it in Nifskope and in the CS/game. The problem is that when my PC walks along the hallway, it is getting blocked by something. It's not the niche (which has a protruding lip) because my PC can get by the block by walking down either side. It's just in the centre that the blockage occurs. I'm sure I did something wrong with the mesh but I've run out of ideas on what it could be. I've tried removing collision by adding the NiStringExtraData block but it made no difference.

I do intend to get more into modelling as my mod progresses but right now I just want to get this fixed.

EDIT: Fixed at last.