I've been on SMP for 7/8 months now - its sucky but that's all there is. We are just about keeping our heads above water. I am very jealous of anyone who gets enhanced maternity benefits! However, I think I remember from previous posts that you are a carer for your OH - is it worth trying to apply for a carer's allowance for you or Disability Living Allowance for him?
I take it you are already claiming child benefit and its maybe worth ringing the tax credits office to advise them of your change in income, although, as far as I understand it, they are unlikely to change your award until the end of the financial year.
Good luck
Valentine Xxx