I thought if you were already a customer you got the £750 payment if you ordered before the last day of 2023. And it was only new customers would get the £750 payment if they ordered 1/1/24 onwards. Could be wrong.
Are you getting confused – the GCB is £750 as described above. The New Vehicle payment is also £750. For existing customers this will be paid towards the AP or into a bank account at next change of car.
You were paid £350 have allready been paid £350 of the GCB, leaving £250 left to paid when you hand your existing vehicle back. Add to the the £750 NVP leaves you with £1000 as @bobscot1 said 🙂