As an Au Pair - recently turned qualified Nanny, I would expect £10ph but depending on terms of contract/agreement as an au pair, I would expect £8ph if babysitting was not clarified at least 2 days in advance. Usually I would do 1 babysit for free during the weekdays. All I would say is. if you go out regularly of a weekend and you have an au pair, be respective of the fact that they also have a social life. If you are going to ask for their time over and above the hours they do during the week, it is only fair to pay them reapectably. As I said, £7-8ph for babysitting is enough for an au pair for extra babysitting.