The present invention provides a compound of the formula: ##STR1## or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof wherein: A is a group of the formula (IA) or (IB): ##STR2## R.sup.1 is 1-hydroxyethyl, 1-fluoroethyl or hydroxymethyl; R.sup.2 is hydrogen or C.sub.1-4 alkyl; R.sup.3 and R.sup.4 are the same or different and are a variety of substituents X is alkanediyl containing 1-6 carbon atoms optionally interrupted by O, S(O).sub.x (wherein x is zero, one or two), --CONR.sup.5 -- or --NR.sup.5 -- or wherein R.sup.5 is hydrogen or C.sub.1-4 alkyl; or X is alkenediyl containing 1-6 carbon atoms optionally interrupted by O, S(O).sub.x or --NR.sup.5 --.