If you were starting from scratch you would lay in a 5 core cable
from
the boiler (Perm L, N, E, Switch Lived, Pump Live). Currently, you
probably have a pair (or even three) 3 core cables on that route.

Sorry, I should have mentioned, I currently have a 5-core wire as you
described. Knowing that is a valid setup, and knowing that there is
such a thing as a pump live connection makes the whole thing make sense
now!

Thanks once again for your help.