having 7 or 8 carbon atoms, e.g. a benzyl or phenethyl group) or a cyclohexyl group, or R.sup.7 and R.sup.8, together with the nitrogenatom to which they are attached, represent a piperidino group, a 1-pyrrolidinyl group or a morpholino group; particularly preferred amongst such groups as those of formula --CO.multidot.NHR.sup.8 (in which R.sup.8 is as defined above); an alkyl group,preferably an