Hi Laura, I think this general complaint form also includes complaints for grants but, you can ask Mobility when you speak to them in the morning. My advice would be to put your initial complaint and any correspondence in writing (email)
https://www.motability.co.uk/get-support/complaints/
If motability tells you your car is ready to collect, after checking any notes they have on your file. I would be taking the matter up with your dealer you got the car from in the first instance, as it could be your dealer has pulled a fast one. Its also worth asking Motability what collection date they have on their sytem, if they say delivery date has changed.
Try to reman calm and take one step at time & let me know what mobility says in the morning 🙂