You may challenge a Coroner's decision or an inquest conclusion. If you are thinking about doing this you should first seek advice from a lawyer with expertise in this area of the law. Some bereavement support organisations may also be able to offer advice.
If you decide to proceed, you need to make an application to the High Court for judicial review of the Coroner's decision or conclusion. You should do this as soon as possible and within three months of the end of the investigation.
There is a separate power which under the Attorney General, or someone who has received the Attorney General's permission to do so, may apply to the High Court for an investigation to be carried out if a Coroner has not held one, or for another investigation if this is in the interests of justice (eg.because new evidence has come to light). There is no time limit for these applications.