The Royal College of Psychiatrists - Guidelines for its members

The Royal College of Psychiatrists’ Report (2015) provides guidelines for use when psychiatrists are giving expert evidence to a court or tribunal. A copy of the Report is available here.

Note page 22, where the Report says:

The expert should not take instructions that go beyond psychiatric expertise and evidence-based data: for example, responding to a request to determine whether an individual is telling the truth, or diagnosing a novel psychiatric condition for which there is no evidence base.

See also CPR 35 regarding duties of experts, and CPR 35.10, when the court (or tribunal which should follow the CPR here) will order disclosure of instructions given to the experts by the lawyers.