I am in a similar situation myself. However, as I had just bought my car from Motability, I did not have the full paperwork so paid full tax at the Post Office. I was going to tax it for just a year but this option wasn't available to me as a full years tax is only £30 anyway. That has delayed the time that I need to go through this myself, so I look forward to hearing how you get on.
Nothing on this board constitutes legal advice - always consult a professional about specific problems
Thanks Mrs H, will keep you posted. In the end I had no choice but to pay my car tax in full at the PO, otherwise I could have risked losing the car I was buying. After seeking advice, I was told by the DVLA & disabledmotoring.org that I could apply retrospectively for my 50% discount in writing afterwards. This is where the problems have arisen. Another update soon hopefully.