SueDonim
I remember the agonies you went through, Ferry, and was truly shocked at the time. I’m sorry you still have ongoing problems. 
I think when making complaints, your starting point has to be pretty much as you say, asking What is my aim here? It could be a variety of reasons such as letting off steam, financial recompense, wishing to see someone brought to book, a desire to stop this ever happening again or simply an apology. I’m sure there are others. Also consider how you would feel in the future if you don’t make a complaint.
My son works in patient safety and complaints are in fact an important aspect of ensuring our services are as safe as possible. There’s a lot of science behind it, in which the patient’s experience is very important.
Only you know your GP practice, though, and what their reaction might be, even though afaik, they’re not allowed to take punitive action against you.
Quite, because I'm not really sure what I expect. It's not going to make me better, and it won't give me back almost 20 months of my life or restore my mobility which is so poor due to prolonged inactivity.
Ensuring others don't have to go through this is paramount I think. I do have a good relationship with the surgery and although there certainly were mistakes made, I always felt they were working in my best interest. Apart from one mistake which could and should have been avoided, a lot of medical treatments are so specific to the patient that I suspect they were only taking a generally prescribed route which in my case, didn't work and from there on there were definitely elements of trial and error.
Thanks for clarifying my own thoughts (!) and for explaining about the handling of complaints. I shall give it some more thought.