https://burton-regan.co.uk/process-server/choose-process-server/
When you choose a process server you need to check that they are capable of producing error free Statements of Service or Affidavits of Service. These documents prove to the Court that the document has been served in the prescribed manner.
In summary, we recommend that you select a process serving firm that also operates as Private Investigators. Try to ensure that they belong to an appropriate Trade Association and have full Professional Indemnity Insurances. They should be established and accredited, perhaps with a qualification in professional investigations in the private sector.