Thanks for the reply - after several hours of fiddling it doesn’t appear to be tied to LOD distance (I’ve tried increasing fblockmaximum up to 500,000 and the level 0/1 numbers up without visible difference), I’ve reran xlodgen with 4/8/16/32 levels all set at quality 0, generated underside at quality 0 & height 990 - nothing makes a difference.
I’ll see if I can do the xEdit thing but a quick look at the mod page makes me think it might be beyond my ability
I don’t see anything similar in mainland Skyrim so I might just have to do my best to ignore it whilst in Solstheim.