Actually, you can work whilst on Mat leave and get your SMP
" Working for another employer
If you work for another employer (employer B) before the birth you can still get SMP from the employer (employer A) who is paying your SMP. If you work after the birth for employer B and you were not working for employer B in the 15 th week before your baby was due, your SMP from employer A will stop on the Saturday before you start work. However, if you worked for employer B in the 15 th week before your baby was due, you may work for employer B without affecting your SMP from employer A.
You can do self employed or voluntary work without any loss of SMP. During unpaid maternity leave, you can work for another employer unless there is a clause in your contract limiting any other work. "
http://www.workingfamilies.org.uk/asp/family_z one/fs_pr1_matrights.asp
Just for interest only If you have 2 part time jobs you will get 2 lots of SMP