You would have been given what is called a Child Reference Number (CRN) when the claim for Child Benefit was made. That number is also used as the National Insurance Number. If yir mum knows her Child Benefit Reference Number, she could contact the Child Benefit Office to-day on 08453 021 444 and see if they will tell her the CRN allocated to you when she claimed the Child Benefit for you.
razza, it's already been posted on here that everyone gets sent one when they are coming up to 16, you need an NI number for more than work, you need it for taxable savings etc. Molly's either lost it or they forgot to send her one.