p, a phenyl group, a trifluoromethyl group or an amino group of formula --Na Rb wherein Ra and Rb are as defined above, provided that, if Ra or Rb represents an aryl group or represents a group including an aryl group, that aryl group is not itself further substituted by a group of formula --NRa Rb ; or an aralkyl group having from 7 to 11 carbon atoms which is unsubstituted or is substituted by a