They are very strict about this up here now!
With my dd 5 years ago, you boiled the kettle, filled the bottles straight away, added the powder then put in fridge and could use up to 24 hours later.
Now you boil kettle, leave for 20-30 mins, add water to bottle then fridge.
You are NOT supposed to add the powder now until you ready to give to baby, as this was the part that was developing bacteria.
I take a bottle out of the fridge, heat in a bowl until room temp, then add my scoops. Bottle only good for 24 hours without powder in fridge, and only good for 1 hour after powder added.
I also heard about the advice to make a bottle only when needed, but my m/ws said this was really unnessesary, and also, you would never be done sterilising, waiting for bottle to cool down etc.
I know my baby would have screamed the house down if I made him wait 30-40 mins on me making a bottle up!