You have to have held your PPL for 3 months before sitting any ATPL exams. If this isn't the case, the CAA will write you(and your CGI) a letter informing you that your application to sit the exams has been refused.
As far as I'm aware, most ground schools will take your word that you hold a PPL for the reasons I've just mentioned but i don't think you will have a reference number (same as your licence number) until your PPL comes back but I could be wrong about that.