My au pair experience is limited to just the one au pair, and that didn't work out. It wasn't a disaster (she wasn't a druggie or a thief), but she just didn't like being an au pair. I think I'd describe her as self-centred more than anything. I've spoken to lots of more experienced mums since then, and feel that I know where I went wrong, and I'm confident now that I'd be able to recognise any problems early on and rectify them.
So, Majorstress, don't be disheartened. There are lots of good au pairs out there!
I see a lot of the same behaviour in your au pair as I had in mine. So if I were in your situation, I'd give her one chance to up the standard of her work, or let her go. I wouldn't talk about money at the moment, because presumably a raise is contingent on her doing good work (otherwise you'd have paid her the £100 from the start). She's also cut back on the hours, so it's only fair for you to cut back on the pay (which I imagine she'd find intolerable).
I'd say that my au pair, despite being 26, was still very much a little girl and she seemed to have no idea of working for an employer and the responsibility that that entailed. I think one of the mistakes I made was in being too nice to her early on, giving her "time to settle in" and not clearly communicating the standards I expected, and it was very hard to up the expectations a few months down the road. I think if I had set expectations right from day 1, then she would have grown up on the spot.
It's normal for au pairs to eat with you, but as an adult, she can choose whether she wants to or not. If she is serious about learning English, she will realise that this is one of the best times to speak with real English speakers. My au pair was picky about food, but everyone I've spoken to says that you should not go out of your way to indulge them - some concessions, yes, but not to the point where you are modifying your meals just to suit the au pair. Again, you have to communicate food expectations with them up front, even before they arrive.
As for London rates - I don't think it makes a big difference where in the country. We live just outside London, and this is a lot harder on the au pair's pocket money than living in London itself because of the cost of travelling in every weekend. You've already taken care of the real costs of London living (accomodation), so you shouldn't need to give her a "London Weighting".
I think you are "safe" on £80 - no one could call you a slave driver. And overly generous by paying for language classes on top of that - especially by paying the full cost up front (some people pay half-cost, some pay a bonus on completion).