ACPO Guidelines
The main principles of the ACPO Good Practice Guide for Computer Based Electronic Evidence
No action must be taken that will change data held on a digital device that could later be relied on as evidence in Court.
If it’s necessary to access original data held on a digital device, you must be both competent to do so and able to explain your actions, as well as explain the impact of them on any digital evidence used in a Court.
A trail or record of all the actions taken and applied to the digital evidence must be created and kept safely and securely. If an independent third party forensic expert examines the processes they should be able to come to the exact same conclusion.
The person in charge of the investigation has the overall responsibility of making sure these principles are followed.