The DVLA website (which is now buried within the 'DirectGov' framework) states that you need to take "the appropriate theory test pass certificate (or confirmation)":
http://www.direct.gov...cticalTest/DG_4022541
I've no idea what 'or confirmation' refers to in that link but it suggests that it might be possible to take the practical test without the actual piece of paper which you received after the theory test. I suggest contacting the test centre, where you took the theory test, to ask if they can provide a duplicate certificate (or alternative 'confirmation') in order for you to take the practical test.
The Driving Standards Agency seem determined to make it as hard as possible to actually contact any individual test centre. You might need to actually walk through their doors in order to get any information or advice. Alternatively you MIGHT be able to get the contact details by phoning 0300 200 1122 (which is the number for booking or changing tests).
Chris