The proper cable to use is to the BT CW13308 specification. Cat5E is
usually

CW1308 ??#

See my correction!

OK because they are both twisted pair cables which is the crucial
thing.

Are they? I have a reel of CW1308 that's not twisted, and when I
looked a few years back was unable to find a spec that said it should be
twisted.

Its not *as* twisted as cat5, but it is still twisted.

Assuming it really is genuine CW1308 specification and not somebody passing
something else off as CW1308, it is what BT use and therefore one has to
assume it is fit for purpose! Likewise if the cable you have is genuine
Cat5e specification that will do as well. Using anything but a proper
twisted pair cable can potentially cause problems. Bell wire is definitely a
no no.

Peter Crosland