Thank you! Glad to see it's not just me! (To be fair, I'd have thought that the 318 middle seats would be used that bit more, because they are a lot more spacious than the ones on 320s and 334s).
I was on one this morning, and sat on the outside of a row of three (leaving the middle empty). As I got closer to Glasgow, the train started to fill up, so I moved over to the middle seat (to at least have a chance of it being used). The woman at the window clearly was not happy about the invasion of her personal space! Even then, no-one used the outside seat! So I ended up looking a wee bit odd.
That portion of the train was at 89% seat occupancy (better than usual, as one of the middle seats was being used). Yet my quick calculations showed that the train was overcrowded (over the 35% threshold) and people were struggling to board and standing around in the vestibules, because there's simply no space for them to move down the train.