Thanks for everybody's advice about this. I have made a decision!
I am going to sack her tomorrow.
The final straw is this - it was my ds's 5th birthday today, and we'd planned to go to the park for a picnic. Some of my friends were coming with their kids including my bestfriend who's just had her first baby, and I was keen to catch up. I thought, Great, Au pair can come along, keep an eye on my 5 year old and one of his mates, while they're on the swings, and I can see how my mate's getting on. "Oh no," says smoking au pair, "I don't feel up to meeting people. I'm too shy, and it's the first day of my period. I'll stay here."
So muggins here ends up chasing after my 5 year old and his mate, didn't get to speak to my best friend and other mates (who have children who sit nicely and don't run to swings till their mummies say they can!!)
Oh and another thing -- as I understand it, au pairs are meant to do 2 nights babysitting aren't they? Well, she announced to me she's off for the wknd to her relatives. One one hand that's fine - she's homesick, gets her out of our hair, but I thought it was a bit of a bloody cheek not to ask if we'd need her for babysitting, no?
Anyway, she is so wimpy my kids - who are hardwork - would run rings round her.
I've already paid £50 towards her flight, and I'll her the £100 for the week (a lot for an au pair, but I wanted someone good.
What I've learnt from this is, an au pair is a bit of a false economy. I think I might get an after school nanny, and a cleaner a couple of times a week instead. When you add food and the general hassle factor, I suspect there's not much in it.
I feel a bit wiser (if £300 poorer - I was still paying my old nanny to help her settle) for the experience ...
And she's still bloody smoking...