I saw an ad once - nights for a year with a family but 3 nights a week. It was triplets so I can definitely understand why the help was needed for a year.
I think it would depend whether it was live out and just coming in for nights or live in for the duration that it's pro-ratad for.
I charge about £100 for a 12 hour night with a bedroom provided so I can nap (baby/ies come in with me) as a one off. A 6 night week would be £500. But I'm not actually in the UK right now so my rates are not necessarily indicative of a city in the SW.