Low income freelance whilst on Mat Leave?

[SlimGin]

I've had a look online but can't find any definitive answers for my question: Can I do low income freelance work whilst on Mat Leave and still be entitled to SMP?

I am taking a year's maternity leave (which ends in September) and my last SMP instalment is in June. Because money is tight I'm thinking about freelance writing/editing jobs that I can do in the evening once DD is in bed. Looking at jobs online, I would earn £40 a piece which relates to £40/week.

I'm confused as to whether accepting these payments would compromise my right to SMP, does anyone know?