Personally, I would be speaking to Motability Operations, not Direct Line, and asking to speak to a senior manager, rather than the call centre operatives who answer the phone. As Direct Line are the new insurers, Motability need to understand if the level of service is unacceptable, especially when the previous insurers were very good.
Previously, when I needed a comparable car rather than a standard courtesy car, I spoke to Motability first and they authorised it, then passing on the authorisation to Enterprise who contacted me to discuss it.