I sort of know the area and agree with the point above that people are often after townhouses with bigger rooms, even if the overall space is OK. Lots of the townhouses in that general area are fairly labrynthine in comparison.
The ensuite off the elongated box room is going to make people not consider it if they wanted a master with an ensuite.
I'm not a fan of grey, but you've got nice warm touches which make it work. I like the shots of yellow, even if the photos don't convey how it would feel irl and the dark floorboards add warmth and make it feel less like breezeblock grey.
And the paved yard is offputting for many. OK, so a typical 4 bed newbuild doesn't have more garden than that, but it's greener. I think, if you could find a closing down homebase, or the like, you might find some more opulent wooden garden furniture on clearance. Maybe a couple of larger (than you already have) planters dotted around - all portable so you can take it with you. And, if you can borrow a pressure washer, use it because the flags look quite green. (And good advice from my parents - give it a blast inside door frames etc to make them look absolutely pristine!)