How to attack a Patent. The law of Obviousness
A common tactic when seeking to infringe a patent is to claim the patent is invalid for lack of inventive step if the invention claimed in it was obvious to a person skilled in the art having regard to the state of the art at the priority date.
It is a common approach, first articulated by the Court of Appeal in Windsurfing v Tabur Marine  RPC 59 (CA) and by the Judgment of Jacob LJ in Pozzoli v BDMO SA,  EWCA Civ 588  FSR 37
The following questions need to be asked on the question of obviousness:
(1) (a) Identify the notional “person skilled in the art”
(b) Identify the relevant common general knowledge of that person
(2) Identify the inventive concept of the claim in question or if that cannot readily be done, construe it
(3) Identify what, if any, differences exist between the matter cited as forming part of the “state of the art” and the inventive concept of the claim or the claim as construed
(4) Viewed without any knowledge of the alleged invention as claimed, do those differences constitute steps which would have been obvious to the person skilled in the art or do they require any degree of invention?
The question of obviousness must be considered on the facts of each case. The court must consider the weight to be attached to any particular factor in the light of all the relevant circumstances. These may include such matters as the motive to find a solution to the problem the patent addresses, the number and extent of the possible avenues of research, the effort involved in pursuing them and the expectation of success. Commercial success can be a relevant secondary indicator of non-obviousness. Like all secondary indications it needs to be kept in its place.