I was just looking at the thread on mobility scooter tax & I remembered that the M6 toll has a pass scheme for mobility users. I've just looked into it & there's an admin charge of £15 for a 3 year pass which allows travel along the toll road. I heard about this from a friend & passed the forms on to a workmate who successfully applied.
Here's the web page:
http://www.m6toll.co.uk/pricing/exemptions.asp
There are some terms & conditions there & the form. I hope it helps someone or a family member