Cate, have you tried fenugreek or even motilium prescribed from your Dr? I had a c-sec and my milk took 5-6 days to come in and I started taking motilium in hospital on day 4. I also had GD and DS had to have some formula top ups in hospital due to me having very little to offer him and his low blood sugars. I was feeding him and topping him up with any EBM I had and then formula as well. After every feed I would express for 20 minutes to encourage supply. By the time we were home he was off the formula top ups and I was just topping him up on EBM if he was still wanting more. It worked, and we have been exclusively breastfeeding ever since.

A friend from mothers group had her bub 4 weeks early by emerg c/s and has been b/f and offering formula tops ups as well. She's been taking something else to help her, fenugreek I think. Now, at 4.5 months old she only has to offer a top up at night and her bub is thriving. It is a lot of hard work, and if you're feeding approx 3 hourly you feel like you are constantly b/f, topping up or expressing. I hope you can find the best balance for you and your bub. I second nonie's advice about calling the aba helpline. Maybe they will have some new suggestions for you?
Good Luck