mess

I've not done payroll before and when I took over for the charity I volunteer for, the previous person had put an employee down on week1 month1. I didn't understand what it was for and just continued on the same basis for several months. Now the employee has left and the P45 shows the week1 month1 code and doesn't show the cumulative pay. The employee has queried it. I can't find out whether I ought to have changed it - HMRC didn't write to tell me to change it. Do I just tell the employee that this is the code I was given and they just have to provide the P45 to their next employer? Does it make any difference to their next employer?