Per Peter Gibson LJ in Sabaf v Meneghetti  EWCA Civ 976 at para 59:
The underlying concept for joint tortfeasance must be that the joint tortfeasor has been so involved in the commission of the tort as to make himself liable for the tort. Unless he has made the infringing act his own, he has not himself committed the tort. That notion seems to us what underlies all the decisions to which we were referred. If there is a common design or concerted action or otherwise a combination to secure the doing of the infringing acts, then each of the combiners has made the act his own and will be liable.