Usually it's our cleaner who babysits. She gets the same £9 per hour that we give her during the day here in London, but usually she'll need to put 3yo ds3 to bed, and make sure the 3 older ones have obeyed lights out ! Also, although i always tell her to sit down and watch tv once ds3 is asleep, she'll usually do a load of washing/ironing, and will have wiped down kitchen surfaces if we need to get out of the house while it's still teatime.
Once, for a midweek babysit, she asked how long we were likely to be. i said not to worry it won't be too late as it's midweek; that wasn't her concern, she was just toting up hpw much ironing she could fit in !
(She told me she's addicted to ironing ..and no you can't have her number !)