If you are using the Basic PAYE Tool to prepare your year end P14 & P35 filing and using HRMC own software to internet file - then you need to add boxes 1b and 1b1 to get the total for the P60 1b box, Earnings above the LEL, up to and including the PT
The difference is due to the £153 which is the difference in the employers & employees threshold
So what to do if you got it wrong in the ones you already filed? - Go into the HMRC filing software, lodge an 'amendment' where you add in the extra amount into box 1b... And then you also have to write to HMRC saying you have updated the numbers - address HMRC Employers Office, Chillingham House, Benton Park View, Newcastle, NE98 1ZZ.
I think there are going to be a lot of people out there with this issue who dont even notice!...