A formwork system for use in slip form casting central cores of high rise buildings provides a rebate form for a rebate at the level of each floor slab which is subsequently to be cast. The rebate form is designed to carry ferrules to be cast into the core at the level of the rebate to provide anchoring points for incorporation of reinforcement associated with the floor slab. The ferrules are carried from the rebate form via plugs which are stripped from the ferrules after casting when the rebate form and main slip form to which it is attached is withdrawn.