Dec 13 2022

Are expert reports worth it?

One question we are asked a lot is whether it is worth spending money on an expert report in a drink driving case. One recent client explained that he had been asking for advice online and had been told by others that expert evidence is a waste of time and money. But, what is the truth?

What is expert evidence?

Before we decide whether expert evidence is worth getting for your case, we need to define what we mean by expert evidence.

Expert evidence is material that helps the court understand specialist, or technical issues that would not be visible to the untrained eye.

Let’s take blood splatter as an example. To you or I, a crime scene covered in blood might just look like a horrific mess but a trained expert will be able to look at drops and splatters to figure out where an assailant stood, where the victim was. The expert can look at the fling off patterns to say how the assailant swung a weapon, which in turn will give an idea of how the assailant held his weapon and whether they acted in a calm controlled way or if they had lost control and were slashing wildly at the victim.

In less dramatic cases, an expert can take information about a person's alcohol consumption and then use a series of proven calculations to work out how much alcohol that person would have had in their body at a particular time. This is called a Blood-Alcohol Concentration (BAC) report.

What is the purpose of expert evidence?

Whether you are accused of drink driving or another offence, the ultimate purpose of expert evidence is to persuade the judge or jury to your way of thinking about your case.

Let’s look again at our blood splatter example. There’s been a death. The defendant says that he acted in self-defence, stabbing an attacker to protect himself and that he did so no more than was necessary. The prosecutor argues that far from defending himself, the defendant was in fact the attacker who lost control and launched a frenzied attack on the deceased. Our blood splatter expert may look at the scene and say that there are great arcs of blood up the walls that were flung there as the defendant raised his weapon over and over in order to slash and stab at the deceased with as much force as possible. In this case, the expert is being used by the prosecutor to persuade a jury that their case theory is the correct one and that the defendant is lying about what happened.

In our less dramatic case, we can imagine a situation where an allegation is made that a person drove badly and a member of the public suspected they were drunk so called the police. The police find the car at the defendant's home and breath test the owner who they find to be over the drink driving limit. In her defence, the defendant tells the court that she drank alcohol once she returned home and was not over the limit when she drove. In a case like this, the defendant will want to instruct an expert to produce a BAC report to prove to the court that a) any alcohol consumed in the 24 hours prior to driving would not have put her over the limit when she drove; and b) that the post-driving alcohol consumption is consistent with the alcohol found by the police during the breath test procedure.

Do I need an expert report?

To answer that question you must first identify the issues in your case. By issues we mean the things that you are fighting with the prosecution over.

Let’s say you are charged with drink driving and are saying that you were not over the limit when you drove. The issue in such a case is your alcohol level at the time of driving and, since there is a presumption that your alcohol level at the time you drove was not lower than it was at the time of your breath test, it is up to you to produce evidence that you were under the limit. The first thing to prove is how much alcohol you drank. You are going to be the key source of that evidence but your account can be bolstered with evidence from friends who were with you and who can tell the court about your alcohol consumption. So, with you and your friend we are able to prove that you drank only one pint of beer before driving and three pints after you stopped driving. But, how does the court know whether that one pint will have put you over the limit or not? We can say “well look, it’s obvious that one pint won’t put me over the limit”, but were are in court and we have to prove what we say rather than just tell the court it’s obvious. This is where our expert comes in. They can look at the reported alcohol consumption and tell the court that, “yes this is scientifically consistent with the alcohol level found by the police”. In this way, we can prove both how much alcohol was drunk and whether or not that would put you over the drink driving limit.

What are the consequences of not having an expert report

Not having an expert report is going to weaken your case and leave you potentially unable to prove a key part of your case. Whether that is fatal to your case or not depends on something called the burden and standard of proof.

The burden of proof is a description of who has to prove something. The standard of proof required is the level to which the burden must be discharged. Typically, it is for the prosecution to prove their case so that the court is sure (we used to say beyond reasonable doubt). However, in many motoring law cases it is for the defendant to prove his or her case on the balance of probabilities. This means that if you have the burden of proving something to the court and you lack the expert evidence necessary to do that then it will be fatal to your case. The drink driving case we talked about earlier is a prime example of this – it is for the defendant to prove that she was not over the limit and so no expert evidence will nearly always mean you cannot discharge the burden of proof and so you will be convicted.

There are situations where the burden is not upon you but you want to get expert evidence anyway. Let’s go back to our murder case. The defence are saying that the defendant acted in self defence and did not lose control or do anything that wasn’t necessary to protect himself. Now, the defence might want to get a blood splatter expert who can say that there is no signs of a frenzied attack to shore up the defence version of events and undermine the prosecution assertions about what happened.


In summary, expert evidence can be extremely important in criminal cases and can make the difference between victory and defeat; acquittal and conviction. The key to deciding whether you need expert evidence or not is in establishing clear issues in your case and assessing whether expert evidence will further your position in one (or more) of those trial issues. If the expert evidence is going to further your position then it is likely to be worth pay for. If it is not going to address an issue and further your case then save your money.

If you would like advice about how expert evidence can help you win your trial, please call us on 020 8242 440 or send us a message via our contact page.