Suzi
Housing Benefit does not work like other benefits., you are given an income allowance, the SDP will increase this, as long as you do not fully erode this allowance then you will receive full HB, so unless your HB is being reduced due to your income and you are not already receiving the SDP, which is unlikely, then you will already be receiving the full rate but if you are still unsure then give them a ring.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems