What is the test for apportioning of liability between two parties in a motor vehicle accident?

The determination of apportionment of fault for the injury is not to take into account the possibility that a person not a party to the action causally contributed to the injury. Apportionment must be determined without reference to the possible complicity of others. To do otherwise would require the court to determine the liability of a person who has not had the opportunity to present his or her case. Such a situation was considered in the case of Maxfield v. Llewellyn, [1961] 1 W.L.R. 1119, [1961] 3 All E.R. 95 (C.A.). In that case, dealing with a personal injury arising out of a motor vehicle accident, Upjohn L.J. stated at p. 1123 that: “…the court would have to make an assessment of the liability of the driver of the motor-cycle, he or his representatives not being present or parties to the action and having no opportunity through his representatives to give any explanation of the accident. It seems fundamentally contrary to the ideas of justice as adminis­tered in these courts that this court should have to assess liability for an alleged negligence which has never been alleged against the party responsible. Furthermore, on looking at the whole section I think it is clear that what the court has to do is to assess the damage between those who have been found liable for it, and if somebody has not been found liable, surely then he must be excluded from the computation.”