You could ask a solicitor for preliminary advice, ie, they could interview you to get as much info as they needed to start with, get the medical and hospital records, and get an expert opinion. After that you would want their advice, probably even a barristers advice too, as to the likelihood of success of sueing the NHS.
Once you start the process you don't have to carry on, and its upto your solicitor and barrister to keep you advised of chance of success all the way along.
This preliminary stage would cost roughly I should think about �3K. You cna aks the solicitor to keep you informed every step of the way of mounting costs so you know the score.
The medical expert report is paramount to the advice a solicitor can give you, the drs treating your mum will be judged on a reasonable standard of care that other drs in the same field would have given. In other words the expert will be able to say on a balance of probabilities if it was likely a dr acting to a reasonable standard would have spotted this earlier, and if so what the outcome would likely have been.
Hope this helps
ps, please don't be apporached by those maniacs in the street offering legal services, go to a proper clinical negligence solicitor, ask them if they are on clinical negligence panels.