Mine was tax exempt about four years ago,
you will need to go to a main post office and get the forms .
Hopefully you will get someone that knows what they are doing
and be helpful unlike my post office which was obstructive .
Fortunately I had contacted someone at the dvla Swansea that was very helpful before
going to my local post office and was armed with a telephone number from the dvla.
Thankfully everything was sorted with a smile at my local post office and asked whether
I wanted six months or twelve !