Thanks all for the responses. It's interesting that so many people have had the milk come in later. I'd read up to 14 days, but was a bit sceptical about whether it actually happened irl. Also, when it takes this long, does it properly "gush" in, like I hear happening for some people? Or does it need more work?
In answer to the queries, tbf I say 1ml, I really have no idea though. He BFs for 5 / 10 mins (latch looks good, and he swallows), then stops, and I try to hand express to check I'm empty, and only express say 0.2ml. I hand expressed a few times when he's been asleep, and it's come out about 1ml.
Yes, have been topping up with formula a couple of days as he was getting less wet nappies, and was getting harder to feed as a bit listless with no energy to suckle bless him. Since I've been topping up, he now just cries inconsolably until he has formula, which I think is better than just being weak. Had all this with my first poor boy, but was less willing to top up then and regretted it in the end. This baby was born 9.7lb! So of course needs more to maintain his weight.
Thanks also for the tips - I tried a lot of these with my first, but it's always helpful to have a refresher. It was lockdown when I had him, so had all the time I the world for trying, but realistically this time I will have a 2yo to consider too, so really need it to come in or not iyswim. Will have a go at a few whilst my OH is on pat leave though, thanks.
There is actually a BF clinic here on Tuesday, so will go and see about IGT too.