Based in Berkshire.
Full of under 5s. Never has been a lot of call for school children where I am. I work 0800-1800 and when I do school runs, I charge 0800-0900 and 1500-1800 at a slightly higher rate, because I hate doing school runs, they use up petrol and the children always eat the entire contents of the fridge. The school children that come school holidays only pay the same daily rate as the younger children.
I don't charge a retainer in the holidays, because I tend to have holiday only children, rather than term time only children.
I think some CMs charge retainers in holidays and others don't. Depends if you want to work really.
I had a parent who came to me for mornings only once and when she realised that I charge a daily rate (hadn't read website, or emails), she thought that was great, because she could work the morning, pop off for a quiet lunch, do her shopping, doctors appointments, etc child free and then pick up whenever she was ready. Others who only want short hours say it is too expensive for the hours they want.
The reason why I prefer not to do half days is because we are out and about a lot and I don't want to be tied to having to be home at lunchtime (although we do come home if a parent wants a late drop off/early pick up occasionally).
I just found when I charged by the hour, parents would gradually cut back their hours and I was left with a lower income and no way to fill the gaps of odd half hours here and there. I now charge for my availability and I'm available Mon-Fri 0800-1800, so that's what I charge.
Hope that helps.