How is Oliver's weight gain going, Sam? I'm guessing that there's cause for concern if Plunket also think there's not enough breastmilk for him? But is he alert (well, as alert as a 3-week-old gets?), does he seem generally OK within himself, or is he not looking too well? I'm asking because as I learnt with Ella, low weight gain can worry a mum sick, but it's really important to look at how alert/active/happy the child is too, because some kids are just small. It's time to worry when they're small and lethargic.
You're already doing everything you can and I think you deserve a big pat on the back and some recognition for that, because it's such hard work. And like others said, babies are far more effective at getting milk from a breast than a breast pump is. Expressing works well on some boobies, not so well on others, and some people find different pumps better than others too. And it's so frustrating because you can't measure what you baby has drunk straight from the breast, so you've only got his disposition and health to try and measure it by. But remember that it's still early days for you, and it seems so many of us had awful struggles with breastfeeding in the beginning. For some it gets easier and becomes really convenient and lovely, and for others it doesn't, but don't feel guilty if it doesn't - it's not your fault at all. And many, many babies have thrived on formula.