A cm can only have so many children at any given time in their care, but I think they're allowed more than one 1 year old on their books so they wouldnt be wasting the slot. They must be because my cm has several, and she's all above board, Ofsted inspected etc.
It's not wasting the slot anyway if they have another child at a different time.
My childminder's always been flexible, there was a period last year when she had an extra child and couldn't keep DS for as long on one of the days, but we worked round it and it was alright in the end.
I've used a childminder on an ad hoc flexible basis for afterschool care for 2-3 hours a week. I now have a babysitter/nanny because it suits us better but I rang a load of different ones and many were not averse to the idea