Lobster wrote:

Thanks. I've never owned any 'neutral' (ie black or now blue) sleeving
- is there any situation in a normal domestic wiring system where I'd
need it?

Yes, anywhere you need live, switched live and neutral in one cable.
Examples:

- to parallel-up two lights on one switch, where the supply feed arrives
at the first light (ceiling rose, etc.) but an unswitched 'live' is
needed at the second rose for looping onwards to the rest of the
circuit. Here it's convenient to run 3-core & earth between the two
roses, using (in old colours) red for unswitched live, yellow (sleeved
red) for switched live and blue (sleeved black) for neutral. The
'switch drop' can then connect in at either rose;

- wiring bimetallic thermostats where a neutral is needed for the
accelerator heater;

- bathroom & loo fans with run-on timer.

--
Andy