I'd want to do this right even if I didn't have to have it inspected under Part P! I've slowly been replacing the wiring in my flat to a proper ring final circuit (previously, there was a spur to a single socket in each room, dating back to 1960). Because of the concrete floors I have been chasing the walls and plastering over the cables. However the final leg back to the consumer unit will involve both cables running parallel along with a 3core+earth cable I have to put in for a new light switch so I can rehang a bedroom door.
Because of limited space, these wires are going to be pretty close, and I'm trying to understand page 41 of the On-site Guide (section 7.2.1 Grouping of cables). With twin+earth, for example, what is "one cable diameter"? Is it the diameter of the conductive part, the diameter of a single insulated core, or the width of the total sheathed cable? Alternatively, what are single-layer circuits? Does that simply mean the cables don't overlap (in which case, I am covered by the second part of 7.2.1(i) because I have less that 5 circuits for this section) or is it something more complicated than that?
Many thanks Stephen