Okay, so now I've worked my way out of the tunnel and onto the nurnies (shields, plaque etc.) on the wall, where I've started adding specular from a secondary light. Does this to address the flatness issues?
I also had a play with replacing my darkest gray with a saturated magenta, which seems to work out really well in the passage (Thanks for that great tip st0ven!). I think the magenta is breaking the tone of the surfaces outside of the passage though, so I'll have to fix that.

Yup, the cobblestones need to be completely redone, and the bricks need to be prettied up.

Thanks for all the feedback!