A friend asked me to have a quick look at the wiring in his garage, he'd mangled a metal clad socket on the wall when pulling his trailer out,
He got a new socket, but when he took the old un off, he found 2 sets of black and a G/Y earth singles wire, one of the black wires had red sleeving on it for about 3 inches,
it is in metal conduit as i imagine you've guessed, and looks professionally done, nice bends, inspection piece every 360 degrees worth of bends and all that, it's just a single ring main, from a small 3 way consumer unit, which is fed from the houses main consumer unit using 6mm T+E clipped to the wall,
The singles look like 4mm sq, and i can see what they've done, prolly had no red left in the van, so used black for live and neutral, and put red sleeves at each junction to show that, in the consumer unit they have also sleeved one set of black cables with red, and they are in the mcb's (1 interior light circuit, one exterior light circuit, and one ring main) so it's all consistent.
He's worried it may be illegal, as someone could tap into the wiring at say a conduit junction, and wonder why there's no live wire, and think both neutrals are that, i don't rekon any electrician with half a brain cell would think that for a second.. and if they did, well, that's Darwin in action for you :)
i did tell him that the alternative is to pull the wires out of the conduits, and replace them with proper colours, which would be blue and brown now, which would mean a licensed sparky, part P, bringing other things upto 17th edition standards and a fairly big bill at the end, and he'd still get the exact same result... which is 6 plug sockets in the garage that work when you plug things into them (if he can learn to steer the trainer around them that is)
i've heard about sleeving a red wire black for a switch circuit, but can you sleeve black red, for a ring main circuit?
i do sometimes wonder if people get by this part P thing by using modern blue and brown T+E, removing the insulation on the live and neutral wires at the outer insulation, and slipping bits of red and black insulation from a few meters of old cable they have.