I bought an endoscope for this very purpose. It's an industrial endoscope, rather than an up-your-endoscope as favoured by medical practitioners and those space aliens who abduct people, and I have to say that it's not bad for £30. What is bad is that the electricial we called to check out the wiring didn't have one. I'd have thought he'd have had enough room in his van for one, given that ours came in a box measuring 13cm by 14cm by 5cm and it's not exactly expensive. The reason the photo has a circle around it is because the camera kept catching on the insulation after 20cm or so, so I had to poke a bendy metre-long tube in first to run the camera through in order to see where the cables I was following went (not-entirely-unexpected answer: the light switch).
The problem with the lighting seems to lie somewhere between the light socket and wherever it taps into the mains. I know this because if mains electricity was reaching the light socket I'd be dead. As for where the cables from the light do tap into the mains, though, well the cables in question immediately dive through a joist so we don't know. My cable-detection device doesn't pick up anything, so we're going to have to drill some inspection holes to figure out what's going on.
At least I know the cables are dead, so if I drill into one I'm not going to give myself an unnecessary heart defibrilation.