Hi @Lexie,
did you order your new car before 1st October 2022? Presumably you did as you’re expecting delivery at the end of December, in which case you should expect the full 600 GCB on the car the you’re returning plus the 250 for ordering a new car during 2022 (850 in total)
My understanding is that the 350 that you have already received is an advance for part of the GCP for your new car. When you return your next car in 3 years time, then you will receive the 250 balance of the GCP (the same as someone ordering a new car after 1st October 2022)
There are several other threads on this topic. Information on Motability website is confusing. If I’ve misunderstood, then I’m sure someone will be along shortly to correct!
Sean
Current car Hyundai Kona Ultimate EV