I love the idea of a nanny or cm but like PP have said the idea of leaving unattended with only one adult is worrying for me. Although I don't for one second think that anything would happen I think it's only natural to worry!
We have a CM duo who work from the same address and are related. I find this perfect because one for the majority of the time there's 2 adults there and children are split into 2 groups based on age giving more flexibility in the activities they can do.
But there's still enough other kids for them to socialise with, while being few enough that in the last 2 years our child has only picked up one minor illness from there. Being around only a handful of other children really reduces how many episodes of illness you have to cover.
There are actually quite a lot of CMs near me with this set up so if you can find one it's a great compromise.
Other advantages: they charge by the hour so we end up paying a lot less than a full day at nursery would be as our children go for 8 hours rather than the 10.5 hours most of the nurseries are open for, but they are also very flexible if one day we need to pick up late or drop early. Also most nurseries near us don't open until 8 which is occasionally too late for us. They are obviously less flexible than a nanny but also a lot cheaper.
There has never been a time I can remember when both CMs were ill and when one has been ill the other has still been able to still look after my DD. If they couldn't take them because they are ill, we don't pay. If our children are ill and don't go we still have to pay but if they have a temperature children are allowed to attend once they've seen a Dr.
We don't pay for holidays they take, but pay full fees for any holidays we take.
On childcare.co.uk you don't have to pay for contact details if the CM has better membership. I found mine there and ignored anyone who I couldn't contact for free.