A polymerisation catalyst composition comprising (1) a transition metal compound of Formula (A), Z being 5-membered heterocyclic containing at least one carbon, at least one nitrogen and at least one of nitrogen, sulphur and oxygen, the others being nitrogen or carbon; M is a Group 3 to 11 metal or a lanthanide metal; E1 and E2 are divalent groups of aliphatic, alicyclic, aromatic or alkyl substituted aromatic hydrocarbon, or heterocyclic; D1 and D2 are donor atoms or groups; X is an anionic group, L is a neutral donor group; n = m = zero or 1; y and z are zero or integer such that X and L satisfy valency/oxidation state of M, (2) a catalyst-activating support which is a solid particulate substance, insoluble in hydrocarbons, comprising at least magnesium and aluminium atoms and hydrocarbyloxy groups containing 1 to 20 carbons atoms, the molar ratio of Mg/Al being in the range 1.0 to 300 and the molar ratio of hydrocarbyloxy groups to aluminium atoms being in the range 0.05 to 2.0, and optionally, (3) an additional activator selected from aluminium alkyl and boron compounds.