The time limits for bringing a claim mentioned here only relate to clinical negligence.
The basic rule is that Court proceedings must be commenced before the 3rd anniversary of the breach of duty.
If the breach of duty is not known of at the time, then the time limit is 3 years from the date of knowledge.
For children proceedings must be commenced before their 21st birthday.
For the mentally disabled there is no time limit, unless the patient has periods when they are not mentally disabled, in which case proceedings must be commenced before those periods total more than 3 years.
In a fatal case proceedings must be commenced before the 3rd anniversary of the death as long as the death occurred within 3 years of the breach of duty (or knowledge of the breach).
It is possible in certain circumstances to have the time limits extended, but no result is guaranteed, and so it is always best to seek legal advice immediately (and more importantly to act upon that advice, because the late litigant has their conduct analysed by the Court when it decides how to exercise its discretion.