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?

Cat5 cable should not be in the same pattress as mains electricity. Only
cables rated to 500V(?) insulation should be layed next to mains cable.
Alternatives are to separate by 50mm, or to provide extra insulation. The
last possibility might work in your case. I suspect running in plastic
conduit would provide sufficient insulation for your needs.

This is not for interference protection, but for electrical safety. I've
never found mains borne interference to be problematic with 100BaseT
ethernet, although I wouldn't be surprised if it was bad for Gigabit.

Will it really make that much difference if I clip them ever-so-gently?

Depends if you cock it up. If you hit the cable with a hammer, then you have
a substantial chance of breaking the extremely delicate conductors. An
alternative is to hit the clip in without the cable present. Then use cable
ties (mucho cheapo) to tie the entire bundle of cables to the clip.

Christian.