Ella
The very first thing you are likely to be offered is a review of your medical notes during labour, delivery and initial post natal on the ward. I understand this is done in a very informative, open and helpful way so that mothers can go through everything that happened, have it explained to then and explained why that lead to decisions that were made, and so on.
I believe the NHS bought this in to reduce complaints, amongst other things. This 'wash up' evaluation may help you a lot in itself and answer many of your questions - when was it noted that the cannula was causing problems, what level of experience did the doctor who put it in have, what was the time delay when the new cannula was put in and what was the reason for the delay... and so on.
This may not help with the post-natal subsequent problems, although maybe you can go through the notes here too? I might help though and I suspect if you place a complaint or ask about it, this will be what they offer you first.
On another subject - did anyone see This Morning earlier? Discussion about the struggles of feeling isolated and not coping easily as a new first time Mum. Suggestions that new mothers need to be more open about these struggles and that women who have been through it before should be more supportive. Interesting discussion to be had in that, I think.