DICEGEORGE was thinking very hard :
I seem to recall reading somewhere that in 2 way switching for house lighting
its a good idea to have a neutral wire parallel to the lives to cut down on
interference, is this so? In which case I need 4 core and earth not 3 core
and earth.

I is mentioned at:
http://wiki.diyfaq.org.uk/index.php/2_Way_Switching

[g]

I think you may be looking at the now illegal 'Carter' method. Yes,
interference during switching can be reduced by running 'flow' and
'return' wires in close proximity, but the difference is slight and
light switching is not frequent anyway. Mostly a home will not be wired
with live and neutral co located along their length anyway.

Network and phone cables use twisted pairs and are used as pairs, so as
to avoid interference generation and pickup of interference affecting
the signal. What is picked up (or radiated) by one wire, is cancelled
out by its opposite number.