Chris (Chester)
are you aware that you can ask Motability to pay the good condition bonus early? If money is an issue and you will struggle to get the funds together to pay the new vehicles AP then Motability will, upon request, review your account and pay you the appropriate GCB payment before you return your current vehicle and have to pay the new AP. The obvious condition to this is that you should be confident that your cars condition upon return will meet the criteria for the payment eligibility and if the vehicle is found to not be eligible due to poor condition then the GCB advance payment would need to be repaid.
Hope this helps
Limpalot