(1) Subject to subsection (2), for the purposes of this Act, in relation to the work which one person is employed to do, another person shall be regarded as employed to do like work if -

(a) both perform the same work under the same or similar conditions, or each is interchangeable with the other in relation to the work,

(b) the work performed by one is of a similar nature to that performed by the other and any differences between the work performed or the conditions under which it is performed by each either are of small importance in relation to the work as a whole or occur with such irregularity as not to be significant to the work as a whole, or

(c) the work performed by one is equal in value to the work performed by the other, having regard to such matters as skill, physical or mental requirements, responsibility and working conditions.

(2) In relation to the work which an agency worker is employed to do, no person except another agency worker may be regarded under subsection (1) as employed to do like work (and, accordingly, in relation to the work which a non-agency worker is employed to do, an agency worker may not be regarded as employed to do like work).

(3) In any case where -