A method of claim 1 wherein the thiazolidine derivative is further selected from compounds where X is a methylene and R1 is of formula VII ##STR60## wherein: R71 represents a hydrogen atom, a (C1 -C25) alkyl group, a (C3 -C10) cycloalkyl group or a substituted (C3 -C10) cycloalkyl group having at least one (C1 -C6) alkyl substituent;R72, R74 and R75 are the same or different and each represents: a