Just wanted to feed back in case anyone looks at this thread again (one of the reasons I started this was because there weren't many threads about debriefs).
Just had my meeting this morning and, to my surprise, it was fantastic. Helpful, informative, and supportive, and the midwife was lovely. She went through mine and DD's notes and talked me through my labour step by step, and was able to explain exactly what had happened and why. Most importantly for me she was able to try and explain why DD was ill and taken to NICU, and although in the end it turned out that there wasn't actually an explanation for her infection, it was incredibly helpful to learn that occasionally mums and babies do develop unspecified infections.
What I found most surprising, given what I expected after the comments on here, was that she willingly admitted that my labour was mismanaged and she was going to bring that to the attention of the teams to try and prevent it happening again. This meant that a lot of things I thought I remembered from my labour but wasn't sure about turned out to be true, which was strangely reassuring. She was shocked that I spent 12 hours in the pre-labour ward vomiting continuously without being given any anti-emetics despite asking repeatedly for them (suffered from hyperemesis all pg) and said that she will definitely bring this up with the team. Essentially, she said that I had become severely dehydrated and this helped slow down my already slow labour, which mean that the infection I seemed to have and which I had passed on to the DD ? an infection which was just one of those things that occasionally happens and was bad luck ? had a lot more time to get a hold on DD. Also, because I was dehydrated I started to get tachycardic, and then DD started to get tachycardic too, and it all snowballed because we were both essentially in distress. I wasn't sure if I remembered the whole being dehydrated bit properly but it turns out I did, which was nice to know!
Also she actually was willing to talk about the next pg, which I thought she wouldn't, and that was a massive reassurance. She basically talked about how further labours are managed and what risks they will flag me up as having (previous c-sec, dehydration and baby in NICU) and how they wouldn't let another labour be slow and they would put me on a drip the moment I started throwing up. I think I want an elcs next time, but it's a huge weight off my mind that if I do have to go for a vbac things should happen which would make it more bearable, and that they now have a records system which automatically flags up these issues the moment I am pg.
I'm hugely glad that I went for the debrief, I feel like a big weight has been taken off my shoulders about why things turned out like they did and about what might happen next time around.