This sure start grant is payable up to 3 months after your baby has been born and 11 weeks before or at any period during these 2 dates.
You need to be in reciept of certain benefits: Income Support, income-based Jobseeker's Allowance,
Child Tax Credit at a rate higher than the family element,
extra Working Tax Credit relating to a disability, or
Pension Credit.
Now you may not be entitled to TC before the birth of your child, BUT when calculating TC they do not and cannot take into account your maternity pay. This means you MAY be entitled after you start Mat leave even wif you weren't before.
You can download the claim for here:
http://www.dwp.gov.uk/advisers/claimforms/sf10 0_print.pdf