Maternity Pay & Tax Credits.

Hiya...In need of some advice.

I'm due to go on maternity leave in july, i currently work full time as does my partner. I will only receive SMP £135 per week and am trying to get some information as to how much tax credits i will receive when my baby is born? I've spoke to tax credits but they won't even give me a rough idea due to it being the new tax year it applies to. My partner works 40 hours per week and is on minimum wage...is anybody already in this situation and can give me some rough figures?

Thanks in advance.