Hi, I've done three pregnancies while doing supply. MA every time. You claim through the Job Centre, you'll need your MAT1B form (from midwife any time from 12 to 20 weeks) and a letter from your agency saying they can't pay you SMP. All info is here.
If you have a contract then it depends how long you have been working on contract. Roughly, you need to have started before you were pregnant to qualify for SMP.