"(1) An industrial tribunal shall not consider a complaint under section 54 unless it is presented to the tribunal before the end of the period of three months beginning when the act complained of was done.
"The third thing we have to say about section 76(5) [of the SDA, Section 68(6) of the 1976 Act] is this. Because it is such a wide discretion conferred upon an industrial tribunal, the task which an appellant has in such a case is a heavy one. Really he must show, if he is to succeed upon appeal, that the industrial tribunal demonstrably took a wrong approach to the matter, or that they took into account facts which they ought to have done, or that they failed to take into account facts which they should have done, or, as a last resort which is always open upon an appeal, that the decision was so unreasonable in all the circumstances that no reasonably instructed tribunal could have reached it."