I can imagine that this has been very stressful for you...
A few years ago (when my children were 4, 6 and 9 years old) my husband was diagnosed with Cancer by the team at St. George's. At the time he was being treated for a DVT and bi-lateral pulmonary embolism as well as being under the cardiac team at St. George's - we've got to know a lot of the doctors there over the last few years! At a terrifyingly scary time, we then got the news that his cancer had been picked up in a scan a few months before the official diagnosis but a technician hadn't been able to read the notes written in someone's handwriting, and the re-scan requested didn't happen. St. George's were totally upfront about it, and did ask us if we wanted to take further action, however, we decided to see how the treatment went and to make a decision when operations etc. were out of the way.
We have been so blessed that the treatement went well and my husband has now been in remission for a few years. His aftercare has been wonderful and we took the decision not to take further action as we didn't want to add to the headaches and significant costs the NHS experiences. I do appreciate that your situation is completely different but our view was that 'we' had been very well cared for and as there was no ongoing issue caused by the delay there was no need to take legal action. We were reassured that a full investigation was carried out and re-training was offered where needed - end of matter.
I am glad that you got good news after the bad, as we did, and I hope that all goes smoothly from now on. I am not sure if this helps you make a decision, but having seen how seriously the hospital took the situation (which after all meant that my husband had cancer for longer than he would otherwise had had it) I hope you feel reassured that they do work hard to try to stop these things from happening. Good luck with whatever you decide