???W-- is (1)???C(Y)-- where Y is a halogen atom, or an alkyl or --XRa group where X is --O--, --S(O)m -- where m is zero or an integer of value 1 or 2!, or --N(Rb)-- where Rb is a hydrogen atom or an optionally substituted alkyl group! and Ra is a hydrogen atom or an optionally substituted alkyl group or, (2) ???N--;