It is almost always better to begin a personal injury claim sooner rather than later.
Compensation probably shouldn't be your first thought, of course. An accident that causes an injury is a serious thing, and there are other priorities - medical treatment, sorting out time off work and sick pay, and so on.
However, once the dust has settled, a legal claim that is started promptly gives it the best chance of a good outcome.
Nearer the time of the accident, you and others who know about your accident are more likely to remember more about what happened. That goes for the defendant just as much as yourself and your witnesses. Whilst insurers acting for defendants are there to defend your case, often if they can get a clear picture of what happened from the defendant's point of view, they will accept the accident was the defendant's fault.
If there is a delay in starting, not only might you and your witnesses be less able to recall events, defendant's witnesses might too. It can also be more difficult to trace those witnesses at all. People move house, change phone numbers, and leave their jobs, and the longer the time that goes by, the greater the chance of something like that causing a problem. Documents like accident records or witness statements can also go missing.
You also only have a limited time to start a claim for accident compensation. In general this is three years from the date of the accident for cases in the UK. It is possible to bring a claim after that time limit, for example if the injured person was a child when they were injured (in which case they usually have until they are 21 to start any court proceedings), or if they do not have mental capacity to deal with their own legal affairs under the Mental Capacity Act 2005. Exceptions can also be made where there is a good reason for not bringing a claim within the usual time limit (for example cases involving child abuse or domestic violence), where the claim can still be decided fairly.
However, although it is certainly possible to start a claim late in the day, it is always better not to take the risk. In some cases, such as claims for accidents involving air travel or ones involving international sea travel, there is a shorter time limit. It can be surprising how far these exceptions extend - for example, the Athens Convention on sea travel would impose a shorter time limit of two years even for a claim for food poisoning on a cruise ship. They are not just confined to sinkings and air crashes.
Another issue is that increasingly, defendant's insurers are becoming more and more suspicious of cases started late. Most of the time, people who go to solicitors later have good reasons for doing so. However, these days when especially nationally-based solicitors are referred large numbers of cases (often by insurers themselves), insurers can get suspicious that a late claim may indicate a case that has previously been seen as weak by a different lawyer, or even one where an insurer has previously refused it and the claimant is having another go.
So-called 'claims farming' (a large number of potential clients being referred to a solicitor, who tries to make the best of things by pursuing the claims that appear to have some merit) can be a murky area, but so too can insurers' responses to it. Here, for example, defendant's insurers are urged to "drive a wedge between claimant and solicitor" by, amongst other things, contacting claimants directly and requiring them to prove their identity and sign to confirm they have actually instructed the solicitor to act for them.
Solicitors, having a strict regulatory regime, would not be allowed to contact their opponents where that opponent has a solicitor, and for good reason. Insurers' regulation is less strict. Although insurers do of course have to guard against bogus claims, there is little doubt that direct approaches to claimants are intimidating, and are meant to be intimidating. The great majority of solicitors' firms dealing with personal injury, including my firm, work properly and for genuine accident victims. If you are approached directly by an insurer when you have a solicitor, you do not have to talk to them, and I would suggest you should not talk to them. Proving your ID or signing some form or other demanded by an insurer is not a prerequisite for making a compensation claim, and never has been.