1-core...
- 6181Y - Grey sheath over red/black insulation.
- 6181YH - Grey sheath over brown/blue insulation.

1-core + CPC...
- 6241Y - Grey sheath over red insulation & bare CPC.
- 6241YH - Grey sheath over brown insulation & bare CPC.
- Obviously black/blue equally used.

6181Y often used with the various "pilot wire" heater systems
(Dimplex, Creda, probably Consort too). I think 6241Y might be
replacing it because the CPC helps trip the RCD side rather than rely
on OC protection, or because it is a bit more mechanically robust
(1.0mm is really wimpy).