Thanks to everyone for the replies. I hadn't even thought of the
electrical safety angle, I was more concerned about mains hum. Just as
well I posted.

I have been using 'dual' pattress boxes from Screwfix (ref 18949)
which take two 1-gang faceplates and have a metal 'partition'
screening each half from the other. However, to make it easier to get
the cables in I've fed them both through one hole and knocked out the
partition, doh.

I guess these pattresses used properly would comply with the regs, is
that right Ross? Do you have other cunning plans which would help me
out?(Incidentally the other cables belong to a 5 amp, round-pin, mains
lighting circuit controlled from the light switch by the door and fed
from a FCU on the ring which I thought I may as well put in while I
was messing about with the floorboads).

Phil, I'm a bit worried about your reported mains hum. All the cat5
cables from the ground floor go up to the attic via a convenient shaft
I found which takes the soil stack up through the building. Between
two floors there is a length of 2.5mm T&E mains cable sharing the
shaft. I can't think of another convenient way of routing the cat5 up
through the building, so I think I'll have to risk it. If anything it
would be easier to find another route for the mains cable. All these
cables are dangling loosely in the vertical shaft, so they shouldn't
be in very close proximity. I think it's just going to be fingers
crossed (no, I can't hear a bee on the line dear).

Ivor, why wouldn't you use cat5 for telephones, am I missing
something?

And as for clipping the things the consensus seems to be that it'll be
alright if I'm very careful, so I think I'll be having another evening
in the cellar very soon.

Martin