What happened when your mother took the case? Was there a judgment? Did she settle the case on terms that the NHS eventually admitted negligence, whatever they'd said before? Or on terms that they did not expressly admit negligence but made a payment nonetheless,just to settle?
There's a number of questions here.One is whether you have a cause of action now. It may be that you can argue that the true consequences following the operation are only now , or recently, apparent. I've a feeling that the rule, whereby you sue on obtaining your legal majority, is rather more aimed at cases where nothing was done on your behalf before. Otherwise, people would always be reviving claims which had failed in the courts, when they were still 'infants' , years and years later, with, in any case, great prejudice to the defendants when witnesses, if they could be found, had long forgotten the events and records or documents had been lost or disposed of. And .'Interest Reipublicae est sit finis litium' 'It's in the interest of the People that there be an end to litigation' as the old maxim had it.
You need a solicitor.