The general rule for injury claims in England and Wales is three years. What that three years runs from is where most of the misunderstanding lives, and it is why people who assume they are out of time frequently are not.
The ordinary position
Three years from the date of the accident. Straightforward where the injury was obvious on the day.
Where the harm was not obvious
Some injuries do not announce themselves. Industrial deafness, vibration injury, gum disease left undiagnosed over years of check-ups. For these, time generally runs from the date of knowledge. That is when you first knew, or could reasonably have known, that a significant injury was connected to what someone did or failed to do.
In dental negligence in particular that date can be years after the treatment.
Children
The clock does not start until an injured child turns 18. In practice a claim can be brought on their behalf at any point before that, and they then have until their 21st birthday.
Where someone lacks capacity
If the injured person does not have the mental capacity to conduct their own affairs, after a serious brain injury for example, the limitation period may not run at all while that continues. This is the exception most often missed.
Fatal claims
Where an injury has proved fatal, the three years generally run from the date of death, or from the date the dependants had the relevant knowledge.
The practical point
Limitation is a hard stop. Once it passes, a good claim is normally lost along with a bad one. But the corollary is that people talk themselves out of claims they could still bring.
If you are unsure, ring rather than email. It takes a few minutes to check your dates, and it costs nothing.