Well, after all the replies you kindly gave - it has come to nought so far anyway.
My P.C decided to go belly up and I have spent the last couple of days re-installing everything. I used this as an excuse to tell her for the time being she couldn't use my connection. I just felt "less safe" her using it while I had no access to anything whatsoever.
As for her getting net access, they are new properties at the back of me and are without any "quick fixes" with regards getting either NTL or BT installed as she honestly wants a temporary measure.
She claims she only uses the net at work and I believe her as she has lived there for over 6 months and not bothered with a phone line so far.
But yes, as some have stated. It would come down to petty things like, what if the router crashed while I wasn't home but she was in the middle of something. What if I was hogging all the bandwidth...etc etc. Too many silly things to argue over.
I have to say that going against NTL's t&c's would have been bottom of my list of concerns, I don't see it as any different than having a lodger and them wanting to use it.
But as I say, things have gone quiet for now as she thinks I still have a knackered PC - which I think I'll be happy her thinking a few days longer.
Oh yeah, she happens to be drop dead gorgeous which is the only reason I even gave this a second thought
that plus she is French and sounds....well, with that accent I'm sure you can imagine :P