Tricyclic heterocycles

A compound of the formula I

• wherein
• X is oxy, thio, sulphinyl, imino, (1-4C) alkylamino or methylene, the last named group optionally bearing one or two (1-4C) alkyl groups;
• Y is carbonyl or methylene, the latter group optionally bearing one or two (1-4C)alkyl groups;
• each R.sup.1 which may be the same or different, is selected from hydrogen, halogeno, trifluoromethyl, nitro, cyano, hydroxy, amino, (1-4C)alkyl, (1-4C)alkoxy, (1-4C)alkylthio, (1-4C)alkylsulphinyl, (1-4C)alkylsulphonyl, (1-4C)alkylamino and di-(1-4C)alkylamino;
• m and n, which may be the same or different, is the integer 1 or 2;
• A.sup.1 is (1-4C)alkylene, (3-4C)alkenylene or (3-4C)alkynylene;
• Het is a 5- or 6-membered heteroaryl moiety containing up to 1 heteroatom selected from nitrogen, oxygen and sulphur and optionally containing a further nitrogen heteroatom, or a 6-membered heteroaryl moiety containing 1 or 2 nitrogen heteroatoms, and which may optionally bear one substituent selected from halogeno, trifluoromethyl, nitro, cyano, hydroxy, amino, (1-4C)alkyl and (1-4C)alkoxy;
• and
• G is carboxy, 1H-tetrazol-5-yl or a group of the formula:
• --CONH--SO.sub.2 R.sup.2
• wherein R.sup.2 is (1-4C)alkyl, benzyl or phenyl, the latter two of which may optionally bear one or two substituents selected from halogeno, trifluoromethyl, nitro, cyano, hydroxy, (1-4C)alkyl and (1-4C)alkoxy;
• or when G is carboxy an in-vivo hydrolysable ester thereof or an amide thereof;
• or a pharmaceutically-acceptable salt thereof.