Played my game on this HB7. Met her through a mutual female friend at a nightclub for about 3 minutes n got 4 IOI's. Added her on Facebook about a week later and then a week after I met her again in college, so i messaged her that evening. Conversation was going well and after 6-7 messages i said "jese i better stay on your good side so! anways im off to bed cos im wrecked! sure gimme your number there and i might send ya a text to chat to ya again.......... might "

She replied the next evening with "haha thanks! ah I prob shouldn't with you being sineads friend ya know? and hey I prob wouldn't get any french work done then anyway, you seem like as much of a chatterbox as me Anyway have a nice weekend "

Any ideas where I've gone wrong?
Obviously I'm not gonna invest that much time in her, but I'm curious, thought my game was tight on this one...

did she used to date sinead? why does it matter if you're his friend?

Sineads a girl, our mutual friend, and I've never had anything sexual with Sinead

It was most likely the words: "jese i better stay on your good side" which lost you the attraction. Women are ONLY attracted to men they perceive as having higher value than themselves : this can be, Genetics , financial, or a myriad of other alpha male qualities.
Your quote presupposed that she had more value than you, and without having any control in the matter the attraction vanished.
This may sound trivial to us guys but to women it's very important and has been hardwired into their feminine brains for millions of years...Consider:

In cavemen days when life was extremley dangerous how far would the genes of a woman go if she was genetically predisposed to be attracted to wussy, weak men who saw her as having higher value than themselves...Not very far. That attraction died out five million years ago.

oh I see. It definitely appears that she was using Sinead as an excuse to not give you her number. Bummer.

I would probably approach this one of two ways. 1. start from the beginning over online messages and take another shot later. 2. host some get together or put together some night out with sinead and people she knows and invite her along. if she doesn't show, you at least did something fun. Her loss.

But to answer your question more directly, I think the last message you sent her was a bad one. As mentioned above, maybe that comment about getting on her good side portrayed low value, though I personally don't think it was that big of a deal. The whole part about "being wrecked" is a terrible thing to say to a woman. Going out and getting wrecked is not really an attractive state. Women like a man who can keep his shit together. But following that up with a number request is also bad. It's like saying "im drunk and horny, give me your number" or that you're only confident when your wrecked. I don't know, who knows what's going through her mind. But you see my point. I'd recommend not messaging when drunk ever, but definitely don't come out with the drunk talk.

Also, I'm unsure about the "......might." on the one hand, it's cocky and funny. On the other hand, women don't like giving out their number in case some guy "might" text them. This may be a little too-cool-for-school.

The message about being a chatterbox leaves a door open in my mind. I would say something like "i'm not a chatterbox, you're just a bad influence. *insert change of subject here*"