I've just found this online:
"If you work for more than 20 SPLIT days it will bring your ShPP to an end.
If your employer asks you to work more than ten KIT days during your maternity or adoption leave you should make sure that your employer will not be treating you as having returned to work and that you can still return on the date agreed or the end of your 52 week maternity/adoption leave period.
In order to avoid any dispute, if you are intending to work more than ten KIT, ask your employer to confirm any KIT days in writing, as well as the rate of pay, and that your maternity/adoption leave will continue after the KIT days.
Once you have worked for more than ten KIT days you will lose a week’s SMP/SAP for any week in which you do some work, even if it’s only for one day. If a week contains, for example, the last of the ten KIT days and another (eleventh) day of work during your maternity/adoption leave, you will lose that week’s SMP/SAP. However, you are still entitled to any contractual pay that has been agreed for your KIT days.
If you have more than one job, your KIT days apply to each job separately. You would only lose SMP/SAP in the week in which you worked more than ten KIT days from the employer paying you SMP/SAP."