Pretty much agree with everything PP have said, I pay £6.20ph and I'm in the SE too. I did see people ranging from £5 up (I went with the most expensive - because I liked her the best, not because she was the most expensive!). I pay half rate for holidays (both mine and hers). I can book up to 4 weeks, and she can book up to 4 weeks, I also pay (half rate) on bank holidays.
For me, I asked for references, basic day, foods, about other children; including how many children she has at once. I didn't ask but maybe should have about additional costs for things (she occasionally does activities which are extra, I have no issue with this but others might).
Again, I personally asked her how she would deal with things discipline wise and how she would deal with my DS who was rocked to sleep (shes got him into such a good routine now I'm jealous, he still acts up for me!).
But in all honesty I walked into her house, watch how she interacted with the children in her care and my DS and liked her straight away. I made the decision to use her by a feeling I got. I haven't doubted my decision at all (he's been with her for a year), she is absolutely brilliant.