A further preferred group of compounds of the invention comprises heterocyclic compounds of the formula I wherein R1 and R5 are each a 1,2,4-triazolyl radical; R2 and R3 are each a hydrogen atom; R4 is a phenyl, phenoxy or benzyl radical optionally bearing a halogen or cyano substituent, and more particularly R4 is a phenyl, 4-fluorophenoxy, 4 fluorobenzyl or 4-cyanobenzyl radical; and R6 is a phe