Pay when starting maternity

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Anyone do payroll that can help me out here,

Had problems with my pay when I sent back to work last time they missed a whole week of Maternity pay and it mucked up my end of year tax/P60 as that was when I went back, just gone on maternity now and looks like they have mucked it up again

Its not my employer but the company he pays so I have emailed him to look into, but need advise,

I worked part time 3 days a week but am paid via salary not days worked, so my understanding it I get paid the salary up until the day my maternity started i.e (salary/365 *number of days prior to maternity date that month) then the maternity pay topping up my salary.

So if my maternity started the 20th of month I would have 19 days of salary pay and then the remaining on maternity pay

Does my reasoning above sound right?
From my workings out I am over £100 underpaid :(