Flagging again this article on NIC which is more useful than either the HMRC agent line or a very well known large employer's overseas payroll bureau (reluctantly, the bureau eventually supplied NIC info for the tax year which was incomplete - unfortunately it stopped at P45 date)
However, the client has several large NICable post P45 bonuses which are clearly not on HMRC's “Information to help complete your tax return.” (Class 2 NICs seem to be wrong too.)
The article suggests contacting the agent dedicated line for multiple employments. The officer clearly didn't know of the facility to check your information on their portal (nor did I at the time). Neither they nor NIC team could supply the appropriate NIC earnings relating to the former employer for inclusion in a tax return. Indeed, the NIC chap said their reconciliations are not complete and they are not in a position to communicate yet.
I've used a worked estimate based on the full period's payslips seen from ex employer - but fully expect HMRC's system to amend the calculation automatically reflecting their incorrect info.
Send copies of payslips next? What are others doing?