In some cases you may require an expert witness. For example, if you are involved in a personal injury claim you may want a doctor to be present to explain the medical issues related to your case. Expert witnesses do cost money, and be aware that you can only recoup a maximum of £750 from the defendant in the event that you win your case. You will need the court’s permission to call an expert witness.
You should do plenty of research to make sure your expert is genuine. Also be aware that witness coaching in England & Wales is illegal. The expert is not allowed to say whatever you tell them to; they must give impartial evidence that they believe to be accurate.
If they do not turn up to court, there is not a lot you can do. You could ask the Judge for an adjournment, but that will mean pushing your hearing back. The Supreme Court recently removed immunity for experts from being sued for the evidence they give to courts, but be aware that you cannot sue an expert for saying something that is factually accurate which happens to be detrimental to your case.
Small Claims Portal has more information on how to get an expert here: https://www.smallclaimsportal.com/guide/59-expert-evidence