A novel olefin polymerisation catalyst comprises: (A) a sub-Gp. III, IV, V or VI metallocene; (B) aluminoxane; (C) a Gp. II, III or IV metal oxide; and (D) an organometallic cpd. of formula (I). Claimed prepn. is by mixing (A) and (B) and then mixing the prod. with a (C)/(D) reaction prod. MR1R2R3R4 (I) M = sub-Gp. III, IV, V or VI transition metal; and R1-R4 = allyl, benzyl, neophyl or neopentyl.