It's a tough decision to make
All I can suggest you do is ask her employer if s/he pays anything on top of SMP as an enticement for you to go back to work - if they do not, then you will be ok in as much as you will not have to pay back any SMP, if they say yes, then you will also know where you stand. Usually you have to go back for a minimum period.
Do you have an HR department? they should have policies set out for this or it may even be in your contract of employment
You can work for someone else when on mat leave so if you have any money making ideas for working form home to top up SMP, maybe you can do that?
Good luck