For the purpose of determining whether an act done in relation to a work constitutes fair dealing, the court determining the question shall take account of all factors which appear to it to be relevant, including—
(a) the nature of the work in question;
(b) the extent and substantiality of that part of the work affected by the act in relation to the whole of the work;
(c) the effect of the act upon the potential market for or the commercial value of the work.