different, represents carbonyl, thiocarbonyl, sulphonyl or phosphoryl (i.e. a group of formula ##STR2## where R3 is a hydrogen atom or an organic group) or a similar acid-forming group; each Z, which may be the same or different, is selected from --O--, --S-- and --NR--, where R represents a hydrogen atom or an organic group; m and n, which may be the same or different, are each zero or 1; and R1