If you have lost your driving theory pass certificate number, there is now a simple way online to find it. Here is a link to the DSA site:-
http://www.direct.gov.uk/en/Diol1/DoItOnline/Doitonlinemotoring/DG_202390
http://www.direct.gov.uk/en/Diol1/DoItOnline/Doitonlinemotoring/DG_202390