Robin
Congratulations on the PIP award being revised, well done.
You will need to contact the DWP office dealing with your claim, you will likely have to complete an IS10 form to confirm that you still meet the other eligibility criteria for SDP, it may be possible to do this over the phone. They should then pay the outstanding SDP from the original date of your PIP Decision.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
