Gill, your hubby would be better to go on sick leave and not return. If the union are going for constructive dismissal against his employers, if he wins his case, he will be paid all or most of his lost earnings on top of compensation - this is especially helpful if the case is long and drawn out - don't forget, unfair/constructive dismissals can take nearly a couple of years before they even come to a formal tribunal.
He may be able to sue for damage to health under the Health & safety at Work Act 1974 - because his employers, assuming they have done nothing about this tyrant in spite of evidence, are failing in their 'duty of care' toward their employees. The union might look at that later though - constructive dismissal claim comes first.
Plus, personal health comes first and it doesn't sound too good at the moment.
Wishing him every success in his claim. Take good care of him