Abstract: This invention concerns compounds of formula (I) wherein E represents hydrogen, C.sub.1 -C.sub.6 alkyl or a group Ar.sup.1 -A.sup.1 ; Ar and Ar.sup.1 when present each represent an aryl group of 6 to 10 carbon atoms or a heteroaryl group of 5 to 10 ring atoms wherein the heteroatoms are selected from oxygen, nitrogen and sulphur, which may be optionally substituted by one or more substituents selected from alkyl of 1 to 6 carbon atoms, alkoxy of 1 to 6 carbon atoms, halogen, haloalkyl of 1 to 6 carbon atoms, haloalkoxy of 1 to 6 carbon atoms, cyano, amino, mono- or di-(C.sub.1 -C.sub.6) alkylamino, hydroxy and nitro; A and A.sup.1 when present are the same or different alkylene groups having one or two carbons linking Ar or Ar.sup.