Previous employer P45 mess up

My previous employer have told me they have sent my P45 to HMRC through the authorised channel, but HMRC tell me they have never received it. However, I can see in my personal tax account online that it says my employment with that company has ended.... so what is going on here? How would HMRC know my employment has ended and yet have never received a P45?

Hi

Your employer would send the P45 leaving details online to HMRC.
It could be that if you have started a new job before the P45 is received the system will update the record to show your old job has ended and the new job started but will not yet show the P45 figures. 

Thank you

the change was october. They would have received the P45 by now. The old employer says they have sent it, and HMRC say they have never received it. HMRC have since told me that because this appears to be something that won't be fixed, I now have to be on month 1 week 1 tax until the end of the tax year.