I had 2 sets of twins, bottles were made by hubby in the morning before he went to work, left to cool and put in the fridge, can't see the problem, all of my children had bottles done like that. It was what we were told to do by the health visitor, they do have a habit of changing their minds and then changing it back again. I'd go for keeping bottles with cooled boiled water and adding the milk when needed, that way the bottles will not take long to do and everyone is happy.