"A finding of fact which is unsupported by any evidence clearly involves an error of law. The tribunal cannot have directed itself, as it should, that findings of fact need some evidence so support them. ... It does not matter whether, with whatever degree of certainty, the appellate court considers that it would have reached a different conclusion. What matters is whether the decision under appeal was a permissible option. To answer that question in the negative in the context of employment law, the appeal tribunal will almost always have to be able to identify a finding of fact which was unsupported by any evidence or a clear self-misdirection in law by the industrial tribunal."