How much tax and NI should i be paying on a smp wage?

I just got my wage slip and im sure i paid too much tax and NI on statuary maternity pay. I got a 7 week pay due to when i went on maternity leave plus it being a 5 week month, (i get paid monthly) So my smp is �113.96 a week, how much tax and NI shouldve been taken of me, it says on my wag slip they took �48.40 tax and �33.66 NI. I think that this seems a little too much for only receiving �113.96 a week. can anyone help me work it out or do it for me. Or is it right? thanks