I'm afraid not; the only HMRC letter I have has a temporary NI number on :/

I have recently been offered a job with a well-known company, but I cannot start induction with them until I provide proof of my national insurance number. They say that ONLY the following count as acceptable proof: P45 P46 Former payslip Letter from HMRC As I have not had any previous contractual jobs, I have no P45 or P46. However, I DO have a former payslip as well as a letter from HMRC. But the problem is that the National Insurance number on them is only a temporary one, so I cannot use them as proof of my permanent National Insurance number. So this leads me onto my first question: why do people keep giving me temporary national insurance numbers despite them knowing my permanent one?? I tried to solve this by using HMRC form CA5403 (a request of National Insurance number confirmation) and sending it to HMRC using a first class stamp. However, reading up on the internet I have read many horror stories of HMRC taking months to get back by post. So this leads me onto my second question: how long does HMRC take to get back to NI number confirmation requests? I cannot start work until I get a letter from them so they are kinda holding my employment prospect in the balance. Needless to say, I am slightly p*ssed off with it all. I would be immensely grateful if anyone could answer my questions, thanks in advance!