I'm in a similar situation, but will be receiving MA as haven't been with current company long enough to get SMP. I've only just been offered the work - sorry to hijack but any guidance here would be really helpful too
SMP is not a benefit, it is a payment your employer is obliged to make. You have to tell your employed if you work for anyone else as an employee, but freelance work is fine. This information is buried away here: "If you do any work in a self-employed capacity during your MPP, then such work will not affect your SMP."
I've been approached to do some freelance design work, and I'm currently on stat maternity leave since 18th May from a permanent position within a retail company. I'm concerned that as mat leave is a form of benefit I could potentially get into trouble over this, can anyone advise if I need to declare it to anyone?