Having replaced a few face plates in the house I've seen a few cables which appear to run horizontally. The wall I need to drill into is a thin plaster board one and I'll be drilling between two sockets on the other side of that. I should be absolutely fine but sods law I could go ever so slightly to far through and clip a random cable. I think I'll just make some small holes by hand and feel about with a screwdriver to try and find any cables.
The magnet and screw thing wasn't for finding cables but is a handy trick for finding studs, nails, screws hidden in walls. I was hoping for some similar trick to find cables.