osa, thanks for the update. Glad you are hanging on in there :)
""B/feeding about every 2.5hrs during the day; 3-4 hrs at night (depends on the night; last night was 3 hrs and then 3.5hrs but the night before she got upset when we tried to wake her after 4.5hrs; we still did and then fed her)."
Not really enough....it works out at about 10-11 times in 24 hours, though the switching and compression is good. I think now is the time to have someone who knows what to look for to watch a feed and assess its effectiveness.
"We have reduced formula top-ups to about 6-7 times a day, i.e. about every other feed; giving in total 100ml of formula per DAY."
That's good progress, and it is justifiable to give formula ahead of a bf for exactly the reason you say - to wake her up.
By now, your baby should be less sleepy - short feeds are within normal, but I get the impresh. she is hard to wake and hard to keep awake, and that's concerning.
"Today is day 12 after birth; went to children's centre locally where midwife weighed DD naked on digital scales. 3kg (vs. 3.35kg at birth)"
This is quite low - the key thing though is is she gaining now? If she has lost a lot of weight and is now on the way up, then it's not so bad. If she is plateauing, or still losing, then that's more worrying.
Are you managing to keep your baby skin to skin/very close most of the day and night? And can you get someone to really observe you?
Hope this helps.