I'm guessing that the Code violation has something to do with either the box not being Earthed or if that 3-Core is a 2-Way switch wire, the green wire should be a different colour.
The green was actually connected to the earth terminal on the box -- I'd already removed it before taking the pictures (it's actually the bare earth conductor with green sleeving on the end).
whats that stray 6491x doing in that box?
For the benefit of non-UKers:
6491X = Single-insulated cable of the type normally pulled into conduit.
The violation has to be the single red wire going up. What was it for? Added three way?
You've got it! The added single was on the switch common with the red/black in the cable used as travelers.
I wish we could get ahold of cable with colors like that here for 240v circuits and switch loops. No more recoloring white wires!
Strictly speaking that black wire should have been remarked red, as black is (was) our neutral color.
We have cable with two red conductors, intended specifically for use on switch loops, although in my experience it's very rarely used.
Unfortunately neither red/black nor red/red will be available for much longer as we change over to the new brown/blue color code.