HETEROCYCLIC DERIVATIVES The invention relates to heterocyclic derivatives, or pharmaceutically-acceptable salts thereof, which possess antithrombotic and anticoagulant properties and are accordingly useful in methods of treatment of humans or animals. The invention also relates to processes for the preparation of the heterocyclic derivatives, to pharmaceutical compositions containing them and to their use in the manufacture of medicaments for use in the production of an antithrombotic or anticoagulant effect. The antithrombotic and anticoagulant effect produced by the compounds of the invention is believed to be attributable to their strong inhibitory effect against the activated coagulation protease known as Factor Xa. Factor Xa is one of a cascade of proteases involved in the complex process of blood coagulation. The protease known as thrombin is the final protease in the cascade and Factor Xa is the preceding protease which cleaves prothrombin to generate thrombin. Certain compounds are known to possess Factor Xa inhibitory properties and the field has been reviewed by R.B. Wallis, Current Opinion in Therapeutic Patents. 1993, 1173-1179. Thus it is known that two proteins, one known as antistatin and the other known as tick anticoagulant protein (TAP), are specific Factor Xa inhibitors which possess antithrombotic properties in various animal models of thrombotic disease. It is also known that certain non-peptidic compounds possess Factor Xa inhibitory properties. Of the low molecular weight inhibitors mentioned in the review by R.B. Wallis, all possessed a strongly basic group such as an amidinophenyl or amidinonaphthyl group. We have now found that certain heterocyclic derivatives possess Factor Xa inhibitory activity. Many of the compounds of the present invention also possess the advantage of being selective Factor Xa inhibitors, that is the enzyme Factor Xa is inhibited strongly at concentrations of test compound which do not inhibit or which inhibit to a lesser extent the enzyme thrombin which is also a member of the blood coagulation enzymatic cascade. The compounds of the present invention possess activity in the treatment or prevention of a variety of medical disorders where anticoagulant therapy is indicated, for example in the treatment or prevention of thrombotic conditions such as coronary artery and cerebro-vascular disease. Further examples of such medical disorders include various cardiovascular and cerebrovascular conditions such as myocardial infarction, the formation of atherosclerotic plaques, venous or arterial thrombosis, coagulation syndromes, vascular injury including reocclusion and restenosis following angioplasty and coronary artery bypass surgery, thrombus formation after the application of blood vessel operative techniques or after general surgery such as hip replacement surgery, the introduction of artificial heart valves or on the recirculation of blood, cerebral infarction, cerebral thrombosis, stroke, cerebral embolism, pulmonary embolism, ischaemia and angina (including unstable angina). The compounds of the invention are also useful as inhibitors of blood coagulation in an ex-vivo situation such as, for example, the storage of whole blood or other biological samples suspected to contain Factor Xa and in which coagulation is detrimental. Accordingly in one aspect the present invention provides compounds of formula (I) (Figure Removed) wherein: A is a 5- or 6-membered monocyclic aromatic ring containing 1, 2 or 3 ring heteroatoms selected from nitrogen, oxygen and sulphur atoms and is unsubstituted or is substituted by one, two or three atoms or groups selected from halo (for example fluoro, chloro or bromo), oxo, carboxy, trifluoromethyl, cyano, amino, hydroxy, nitro, CMalkyl (for example methyl or ethyl), CMalkoxy (for example methoxy or ethoxy), Ci^alkoxycarbonyl, CMalkylamino (for example methylamino or ethylamino), di-CMalkylamino (for example dimethylamino or diethylamino) or aminoCi^alkyl (for example aminomethyl or aminoethyl); the 1,4-phenylene ring of a compound of formula (I) is either unsubstituted or is substituted by one or two substituents selected from halo, trifluoromethyl, trifluoromethoxy, cyano, nitro, CMalkyl, C2-4alkenyl and C2-4alkynyl, from the substituent -(CH2)n Y1 wherein n is 0-4 and Y1 is selected from hydroxy, amino, carboxy, C^alkoxy, C2-4alkenyloxy, C2-4alkynyloxy, C|_4alkylamino, di-Ci_4alkylamino, pyrrolidin-1-yl, piperidino, morpholino, thiomorpholino, 1-oxothiomorpholino, 1,1 -dioxothiomorpholino, piperazin-1-yl, 4-Ci_4alkylpiperazin-l-yl, Ci_4alkylthio, C^alkylsulphinyl, C^alkylsulphonyl, C2-4alkanoylamino, benzamido, C't^alkylsulphonamido and phenylsulphonamido, from the substituent -(CH2)nY2 wherein n is 0-4 and Y2is selected from carboxy, carbamoyl, Ci.4alkoxycarbonyl, N-Ci_4alkylcarbamoyl, N,N-di-C].4alkylcarbamoyl, pyrrolidin-1-ylcarbonyl, pipeddinocarbonyl, morpholinocarbonyl, thiomorpholinocarbonyl, 1 -oxothiomorpholinocarbonyl, 1,1-dioxothiomorpholinocarbonyl, piperazin-1-ylcarbonyl, 4-Ci.4alkylpiperazin-l-ylcarbonyl, Ci_4alkylsulphonamidocarbonyl, phenylsulphonamidocarbonyl and benzylsulphonamidocarbonyl, from a substituent of the formula -X3-L2-Y2 wherein X3 is a group of the formula CON(R5), CON(L2-Y2), C(R5)2O, O, N(R5) or N(L2-Y2), L2 is Ci_4alkylene, Y2 has any of the meanings defined immediately hereinbefore and each R5 is independently hydrogen or C^alkyl, and from a substituent of the formula -X3-L3-Y' wherein X3 is a group of the formula CON(R5), CON(L3-Y'), C(R5)2O, O, N(R5) or N(L3-Y'), L3 is C2_4alkylene, Y1 has any of the meanings defined immediately hereinbefore and each R5 is independently hydrogen or Cj^alkyl, and wherein any heterocyclic group in a substituent of the 1,4-phenylene ring of compounds of formula (I) optionally bears 1 or 2 substituents selected from carboxy, carbamoyl, Ci^alkyl, Ci^alkoxycarbonyl, N-Ci-4alkylcarbamoyl and N,N-di-Ci.4alkylcarbamoyl, and wherein any phenyl group in a substituent of the 1,4-phenylene ring of compounds of formula I optionally bears 1 or 2 substituents selected from halo, trifluoromethyl, cyano, Ci^alkyl, C2-4alkenyl, C2-4alkynyl, C^alkoxy, C2-4alkenyloxy and C2-4alkynyloxy; BisCHorN; the heterocyclic ring containing B is either unsubstituted or is substituted by one or two substituents selected from hydroxy, oxo, carboxy and Ci^alkoxycarbonyl; or one of the following: -(CH2)n-R, -(CH2)n-NRRi, -CO-R , -CO-NRRi, -(CH2)n-CO-R and -(CH2)n-CO-NRRi; wherein n is 0, 1 or 2, preferably n is 1 or 2; R and R1 are independently selected from hydrogen, C^alkyl, C2-4alkenyl, C2-4alkynyl, hydroxyCi^alkyl, carboxyC^alkyl and Ci.4alkoxycarbonylCi.4alkyl or where possible R and Ri may together form a 5- or 6-membered optionally substituted saturated or partially unsaturated (preferably unsaturated) heterocyclic ring which may include in addition to the nitrogen to which R and R' are attached 1 or 2 additional heteroatoms selected from nitrogen, oxygen and sulphur; D is 2-indolyl, 2-benzimidazolyl, 2-benzo[b]furanyl, 2-pyrrolo[2,3-b]pyridyl, 2-furo[2,3-b]pyridyl or 6-7H-cyclopenta[b]pyridyl and is unsubstituted or is substituted by one, two or three substituents selected from halo, trifluromethyl, trifluoromethoxy, cyano, hydroxy, oxo, amino, nitro, trifluoromethylsulphonyl, carboxy, carbamoyl, Ci-4alkyl, C2-4alkenyl, C2-4alkynyl, Cj^alkoxy, C2-4alkenyloxy, C2-4alkynyloxy, C^alkylthio, Ci^alkylsulphinyl, Cj^alkylsulphonyl, Ci^alkylamino, di-Ci.4alkylamino, Ci^alkoxycarbonyl, N-C]_4alkylcarbamoyl, N,N-di-Ci.4alkylcarbamoyl, C2-4alkanoyl, C2.4alkanoylamino, hydroxyC^alkyl, Ci.4alkoxyCi.4alkyl, carboxyC^alkyl, C i_4alkoxycarbonylC i .4alkyl, carbamoylCi.4alkyl, N-Ci^alkylcarbamoylC \.4alkyl, N,N