Only needs a label if there is scope for confusion.
So the circuit breakers need labelling, but the box with them in is full of indentical brown wires that may or may not benefit from an ident - if it is very clear on looking at it exactly where it goes, no need to label - however if it would be impossible to tell otherwise then labels become needed.

If for example live and neutral were not the normal colours, then the individual cores would also need identification as L and N (some cables have all cores the same colour for example - not prohibited but must be lablelled). Or if an unusual combination is used (such as blue for a switched live insteda of neutral is very common - so a brown flash or sleeve on the blue identifies it as not the  normal neutral use of blue)
This article may help explain

https://professional-electrician.com/technical/identification-electrical-wiring/

Mike.

Also sometimes it's adequate to identify by the labelling on the terminal rather than the wire - e.g. in CUs all  the blue wires into the N bar can be identified as to which circuit they belong by the position in the N bar corresponding to the position of the corresponding MCB. Their function (e.g. N) is indicated by colour.

   - Andy.

It is however only fair to say that there is considerable variation in "house styles" between electricians. Some label up everything, while others produce installations that require a fair degree of clairvoyance to understand exactly what  was presumably 'obvious' to them, and not requiring of any further clarification....

Mike

Assuming a correctly  functioning installation or circuit.

Surely their function is indicated by the fact that they are in the N bar whatever their colour.

Would that also apply to things like switches and sockets?

That is, a conductor in a terminal labelled 'L' will be line regardless of its colour.

Hamish Anderson said:

What is interpreted from wiring regulations on BS7671 Section 514.3.2? Do all cables essentially need to have a cable identification label a the core?

Only at terminations for the function but not the circuit (or cable) identification ... but preferably thought the length. A single-core cable, however, could be identified at the sheath and/or the insulation. See Reg 514.3.2.

Hamish Anderson said:

Or is the colour of the cable itself the identification for example brown as live? Or should this be labelled? Say the cable is called "P001" and the live should have to be labelled as "P001-L1"?

Regulation 514.3.2 only requires identification of the function of the core in terms of electrical hazard, i.e. Line (be that L1, L2 or L3), Neutral, Protective Conductor, etc. as per Regulation 514.3.1. As per Reg 514.3.1, the identification does not have to be by colour, but could be by either alphanumeric scheme shown in Table 51, or alternatively if the cable has numbered black cores for live conductors, just the number can be used.

The circuit number or cable number (not specifically required by BS 7671 to be applied to the cable itself, but useful particularly in large or complex installations) can be applied to the cable as a whole, rather than "per core".

So, whilst the cable sheath might have 'P001' applied, the conductor still only needs to be identified by colour, 'L1' (in this example), or by the number of a numbered multicore cable.

If cable numbers were to be applied to single core cables would, perhaps it could be asserted by definition, effectively have the cable number as well as the function, and in this case, if the circuit or cable number were applied, I'd expect to see 'P001-L1' (as per your example).

I'm much less of an expert than those above but.......

Keep in mind that this regulation also is trying to allow for more than just standard circuits from a distribution board - such as wiring up a star/delta motor for example, where I've often used a multi core SWA cable where every cable inside the SWA is coloured all black with white numbers on, or, more commonly, all white cables with black numbers on. 

Three cables for the star connection, three for the delta connection, I'd always choose to take a spare neutral even if its not required, if possible - at least between motor control centre and the local isolator, earth  and then at least a pair that indicates a volt free status of the isolator for the BMS. So 10 cables - all white with black numbering on them. All potentially dangerous, all may or may not be live. Numbered at both ends. I think this would forfil the regulation.

Ideally each cable would be labelled U1, V1, W1 and W1 V1 U1 and N and E and something completely different for the volt free pair. 

Pumps and motors often utilise numbered cables rather than coloured. These type of installations often come with drawings and wiring diagrams to show the numbers use.