Firsty as you are probably aware, there is a statute of limitation on personal injury claims of three years from date of injury or date of knowledge whichever is the sooner, so in the first instance you are time barred.
Folliwng on from that, you probably signed of the case when you received your compensation as full and final settlement which again works against you.
However, if the ongoing problem is as a result of a mis-diagnosis, then there may be a claim for medical negligence if you have only recently found out that the problems you are experiencing are directly attributable to the original accident.
To prove a medical negligence claim, you would have to establish that the actions of the medical people were unreasonable and given the passage of time and how medical practice has moved on, this may be difficult.
I understand your frustration as I had a bad accident on a Police bike many years ago, my case was settled very quickly (long before I worked in persoonal injury law) and because of mis-diagnosis, my claim was undervalued by around Â?250,000 (for reasons which will take far too long to explain here) but because I signed full and final settlement, I was unable to go back later, and too much time had passed to make a clin neg claim.
It may be worth you speaking to a medical negligence specialist solicitor, but don't hold your breath
