Thanks for the support and suggestions.

Yesterday ended up with three top ups - after the second, third and last feeds of the day. On the plus side she is taking longer to drink the bottles - she would inhale them in 5 minutes but is now out to about 12 minutes.

She's just finished her first feed of the day - it is generally her fastest feed and I haven't needed to top her up afterwards for about a month. If the last few days are anything to go by, I'll be feeding again in an hour