As David said, you will get the remainder of the GCB (£250) soon after taking delivery of your new vehicle plus a £750 new vehicle payment.
If you did not recieve and early payment of the GCB (£350) you will recieve £600 soon after delivery of your new vehicle.
If you have already recievedd £250 of the new vehicle payment due already having a new lease, you will get the remaining £500 from late February onwards.