tbh, if there are no other factors, like cost, to worry about, I would go for the school nursery. You know there is a place, and it would secure you a vacancy with the childminder. Once she starts real school all the suitable childminder palces might have been filled by people who got in at nursery stage. Also, a possible smoother transition to school if she is already in the nursery there?
How old is your dd1 yorkiegirl? Shouldn't see why there would be a problem, you may lose any deposit you have paid, I think really you have to do what is best for your dd and try not to feel guilty. If they had their 20 and you pulled out it might be more akward, but it sounds like they are a long way off yet.
I'd agree with everyone else. My dd2 has just started nursery at the school she'll be going to in a year's time, and she really likes being with children of the same age or a bit older. The other advantage of this solution is that your dd would get used to being around the school that she'll be going to in 2006, so she won't have a big transition to deal with then. I don't think that the offer of a place is binding in any way... It's not as if you've paid a deposit or anything. I'd just spin it that your circumstances have changed because you've managed to get your dd a place with this childminder, and you want her to get used to being in the school that she'll go to in 2006, and not have to have her moving again in another year (and then maybe not be able to find a childminder).
When is she supposed to start? Have you signed the contract or agreement? i wouldn't have thought they could cause a problem since they are not actually able to guarantee that they will have enough children to function!
IIRC in order for them to claim your governemnt grant you have to fill in the form that will be issued on the headcount day in September. They can't sign the form for you IYSWIM so you wouldn't lose out on the grant if you went to the other pre-school.