Expert Witnesses must always be aware that their primary job is to assist the Court and not to be an Advocate for the party that has retained them.  For a Court to be able to rely on the evidence of an Expert Witness the following guidelines will be of assistance:

1.  An Expert Witness should at all stages provide independent assistance to the Court.
2. The Expert Witness should not give evidence as to what the expert would have done in similar circumstances or otherwise seek to usurp the role of the judge.
3. An Expert Witness should co-operate with the expert of the other party.
4. The expert evidence presented to the Court should be, and should be seen to be, independent.
5. An Expert Witness should state the facts and/or assumptions upon which their opinion is based.
6. An Expert Witness should make it clear when a question falls outside their experience.
7. An Expert Witness must say explicitly if they think their conclusions are based on inadequate facts.
8. An Expert Witness must be ready to reconsider their opinion.

Point 2 presents an easy trap to fall into in relation to assessing the conduct of a professional.  The real question to be addressed by the expert is whether the professional acted in accordance with a practice accepted at the time as proper by a responsible body of opinion skilled in the relevant field (Bolam v Friem Hospital Management Committee (1957) 1 WLR 582).