Neil, my van has disabled tax and it is all done online after the first time, no need to show insurance, MOT although you have to show proof that you qualify for disabled tax, very easy to do.
The first time it has to be done at the Post Office, (unfortunately, why make a disabled person travel to a Post Office?

) where you have to show the letter for proof that you qualify, the next time that it is due you can do it online, they simply ask if you still qualify, all they need for proof then is your National Insurance number.
I would get down to the Post Office one way or another, no need to wait for the six months to be up either, sorn it then get to the Post Office to tax it from April, you will receive a refund for any months left that you have paid for.
Interestingly, if you don't qualify for the enhanced rate of mobility allowance but only get the standard rate you still qualify for 50% reduction in vehicle tax, this can be a good saving.