New employer not deducting student loan

After finishing my degree I got a job that I did for 2 years, during which time my earnings did not reach the student loan repayment threshold and so no repayment was made.

I now have a new job and have relocated to a new city. My new earnings are well over the repayment threshold. I have now been paid twice (monthly) and the student loan hasn’t been deducted. I handed in my P45 but I have noticed that the box that says ‘Student Loan Deductions - Student Loan Dedcutions to Continue’ is blank - presumably because I never paid the loan at my old job?

So, what steps do I have to take to make sure my new employer starts deducting the student loan? - I understand that this would have to be a notification from HMRC advising them to start deducting it, so do I just assume that they will eventually do this? Also, once they have notified my employer and the deductions start, will they be backdated to when I started my new job?

Thanks