I have contacted the Motability people direct and received this reply:
"Thank you for your email asking if your sister's fiancé got a car on the Scheme with you being his designated driver, would you be able to use the car without him being in the car to go to and from your place of work.
A Motability Scheme car must be used for the benefit of the disabled person whose allowance is paying for it. Provided the disabled person is deriving material and/or practical benefit from the car and has access to it when required, they do not necessarily have to be present every time the car is used. As the designated driver, you would need to live within 50 miles of your sister's fiancé."
I assume this means that I could take the car to work and for my own personal use as long as he has access to it when he needs it.