@testy1997 broadly good. I was worried as he has bad reflux and frothy poos from feeding issues so I had images of him screaming vomiting and pooing, however I literally had no choice as I couldn't physically keep up the rocking. In the end it was fine. It's not nice to hear him cry but it worked from day one. About an hour to get to sleep the first time, with peaks and troughs of crying. A few times I thought no this will never work and then he'd calm down and nearly fall asleep before I went in so i left him. Once asleep, he woke and cried much less at 2am, then woke for the day at 6.
Since then, he does usually cry a bit before sleep but it tends to be either 1-2 minutes of furious screaming and then sleep (when he's too tired I think), or 10-20 minutes of moans, babbling, complaining etc.
Naps have been much harder and we've had to set boundaries of how long we will actually leave him for if he's not sleeping ( aiming for an hour if we know he definitely needs to sleep, 20 mins if he has slept then woken up early).
He woke and cried for quite a while on and off last night at 3am, which is roughly when I usually feed him so probably habit but I can't help thinking he's hungry (he could definitely go longer without food but he's not in the habit) so went in and fed him when he stopped crying for a few moments. We are just trying to decide whether to go in when he cries at night now because it's for a reason, or whether that will undo all the work we've done...very hard to say! I don't believe he's ready to go overnight without food but that's what our plan was aiming for.
Overall it's been life changing. I have a happier baby, happier me, happier partner. I don't believe for a moment the people that say they learn not to cry for help - he very clearly does continue to cry if he has a dirty nappy or is hungry or whatever. Hunger is a tricky one to provide without encouraging frequent feeds, but other times when he has woken up and not gone back to sleep reasonably quickly, and been crying, he has had a dirty nappy and has gone to sleep when we change it.
Also, the person who suggested a fixed wake up time is right I think - seems to be one consistent piece of advice across all styles of sleep training. We let it go to pot one day last week and it did seem to cause chaos the rest of the day!