(1) Where, in any action brought by one person in respect of a wrong committed by any other person, it is proved that the damage suffered by the plaintiff was caused partly by the negligence or want of care of the plaintiff or of one for whose acts he is responsible (in this Part called contributory negligence) and partly by the wrong of the defendant, the damages recoverable in respect of the said wrong shall be reduced by such amount as the court thinks just and equitable having regard to the degrees of fault of the plaintiff and defendant: provided that -

(a) if, having regard to all the circumstances of the case, it is not possible to establish different degrees of fault, the liability shall be apportioned equally;

(b) this subsection shall not operate to defeat any defence arising under a contract or the defence that the plaintiff before the act complained of agreed to waive his legal rights in respect of it, whether or not for value; but, subject as aforesaid, the provisions of this subsection shall apply notwithstanding that the defendant might, apart from this subsection, have the defence of voluntary assumption of risk;