hey, what a coincidence. i didn't know you were a Brit!
i'm just asking these questions for future reference, by the way, for when i DM a game. you're welcome to arbitrate it how you wish, or even hand-wave it. that's probably what i would do if it came up, but i was just wondering what the RAW had to say about it, and i can't find anything.
i did find that Spot checks suffer a -1 penalty per 10 feet... but 10 feet beyond what? the illumination rules don't cover natural light, only torches and the like. the terrain rules don't really cover urban settings, aside from well-lit thoroughfares (which this isn't). and what's the base Spot DC for someone who isn't actively hiding, anyway? i dunno, this situation just brought up a couple questions about rules arbitration, and each of those questions brought up a few more.
what did you apply the -2 penalty for? would Bo suffer the same penalty if he didn't have darkvision?