In this case the EAT sets out important guidelines on expert evidence in employment tribunal cases.
Employment Appeal Tribunal, 2001.
The EAT’s judgment is available in full on the EAT website at www.bailii.org/uk/cases/UKEAT/2001/1438_00_2003.html. The guidelines are at paragraph 36 (bailii.org) of the judgment.
For example, the EAT comments that expert evidence will not always be admitted. A prudent party will first explore with the employment tribunal at a Directions Hearing or in correspondence whether, in principle, expert evidence is likely to be acceptable.
Also joint instruction of an an expert is the preferred course (though sometimes each party will instruct its own expert).
The EAT makes many further points, and I suggest looking at the judgment.