Hi,
Actually good timing re au pair meet-ups, as my original post about lessons was for our short-term summer au pair, who's now been and gone, but our new au pair for the school year has just joined us and would also like to meet other au pairs in the area! I'll message you.
Re English lessons if it's helpful to anyone else, Richmond English School in Mortlake turned out to be really good:
http://richmondes.com/home.
They have special au pair lessons in the middle of the day, evening or weekend, and are really flexible so you can miss a day if you need to stay home with a sick child for example, and take the class on another day instead. They also do meet-ups etc for the students. Our au pair really enjoyed the classes and studied towards the Cambridge Advanced Cert - although she did say the main teacher, Tom, was fantastic, but pupils weren't so impressed with a couple of the ad hoc teachers and were making sure they took his classes.