Some great advice on here
@HM, I know it can seem like a lot but I do think there is great advice in these posts.
You don't want her to come over and feel resentment that she's the poorest au pair in the room. One of my neighbours had their au pair poached by another neighbour for an extra thirty quid a week and that caused quite a bit of tension on the street
Try not to look at it as "four hundred quid" but more an extra "one or two hundred quid" over and above what you were planning to pay, I think most of us in Nappy Valley can find that money.
A great au pair is wonderful but it is, as someone has already alluded to, a bit of a mindset and if you're really keen to keep costs down and that's an over-riding priority then it may be that an au pair isn't the right choice for you.
I'm not saying that I believe that is the case, just if you really feel that the extra hundred pounds a month makes it non-viable then it may not be the right choice.
Good luck in whatever you choose, you've obviously given it a lot of thought which is important