We have (almost) always had good luck with LikeMinders, which we used quite intensively years ago, and only once recently.
The "almost" refers to a couple of occasions in the past where the nanny didn't seem particularly engaged with the children, but I had no concerns for their safety, so as it was a matter of covering half a day in an emergency I wasn't too fussed; however, the majority of nannies we had from them were great and broke the ice with the children straight away.
They were quite good at last minute bookings.
Their website explains all the fees; I believe you have to book them for a minimum of 3 hours and you pay a fixed booking fee in addition to the nanny's fees (which you pay to the nanny directly). If you think you might need them more or less regularly it's worth paying the monthly subscription as the one-off fee is quite steep.