Martin Pentreath wrote:

At the moment I have some points where the cat5 cable emerges into the
same double patress box as mains electricity. Is this going to cause
me problems? If it does will it only be a problem if I try to shove
several megabits of data down the cable, or will it interfere with

The problem is that if you manage to feed mains into low
voltage cabling things will get both dangerous and expensive.
Connect mains to the public telephone network and you'd best
consider yourself lucky if all you get is a large bill.

telephones too? I could put in separate patresses, but I'm fed up of
chiselling holes in the wall.

Either get used to it or use some surface mount boxes for
the data outlets.