Some cases (such as Clinical Negligence and Construction disputes) inevitably require the use of expert evidence.  It is preferable that the respective experts should have discussed prior to the mediation where they agree, where they disagree and why they disagree.  In my experience, it is unusual for an expert to alter his or her opinion at a mediation.  Indeed, frequently an expert at a mediation appears to want to take on the role of an advocate for the party instructing them, which in turn often hardens the other side's expert's resolve, and parties increasing become entrenched.

Rather than incurring the cost of having an expert attend the mediation, it may be preferable to have the expert available to take telephone calls if necessary.