Per Mustill LJ in Unilever v Gillette  R.P.C. 583, at p.609:
I use the words common design because they are readily to hand but there are other expressions in the cases, such as "concerted action" or "agreed on common action" which will serve just as well. The words are not to be construed as if they form part of a statute. They all convey the same idea. The idea does not, as it seems to me, call for any finding that the secondary party has explicitly mapped out a plan with the primary offender. Their tacit agreement will be sufficient. Nor, as it seems to me, is there any need for any common design to infringe. It is enough if the parties combine to secure the doing of acts which in the event prove to be infringements.