You have to differentiate not attending mediation and not attending the tribunal.
The former is not compulsory and you would be entitled to write to Fair Trading and say you were not attending because the complainant was using Fair Trading as a form of harassment by wasting committee time (listing all the previous failed cases).
For the Tribunal, I would be tempted to hire a lawyer to defend your position while making it clear before the hearing that you
a) would be demanding costs from the litigant and
b) would seek to have him declared a vexatious litigant by the Tribunal