Hi Bryando,
I have experience of this, and it's very time taking and not easy. Because of how long it takes (normally 5 weeks) after receving your car tax renewal form, the DVLA don't normally get your cheque on time, meaning that your vehicle can be left untaxed for a few weeks. I have found the best thing to do is to pay your car tax up front online, and then send your PIP award letter, V5C document and a letter asking for 50% back to be the best way. You always get your documents sent back within a couple of weeks, but the cheque usually follows a week or two later. I've tried sending 50 percent payment twice in the past, for it to either be rejected because of lack of documentation, or my vehicle to be left untaxed and possibly taken away and crushed because of the delays when applying.
Hope this helps, Chris.
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
