Ah-ha thank you, I think I'm aware of what happened :)
I just realized that HD Enhanced Terrain released an update AFTER the STEP 2.2.7 guide was put up, so I installed the wrong one! The correct file is listed under 'Old Files' on the nexus.
I also downloaded the Skyrim HD and Serious HD files separately from Nexus, so maybe that could explain it, but I had another check; Skyrim HD - Dungeons is completely overwritten by Skyrim Realistic Overhaul, and a couple files by Re-Defined Dungeons, Serious HD Landscape by SRO and Terrain Bump (mostly), and Serious HD Riften by SRO. Was I supposed to just use the Landscape textures from SRO?
Thanks again!