The grant is only available to those on benefits. People on the normal tax credits won't be eligible I'm afraid. Already checked this out myself with the job centre - the criteria has always been the same.
You can get it if you are on normal tax credits. You have to get a rate higher than the family element, which is about £545 a year, so if you get roughly over £10 ish a week of child tax credit then you are entitled to it. I got it for my last 3 children & we are not on any benefits, we do get child tax credit though. However from April it is being changed so that you will only get it for your first child, this was announced in the budget last June. So if this isn't your 1st baby & you are 29 weeks in April or later you won't be entitled.
Also, a lot of first time parents aren't eligible to claim before their baby is born as they aren't claiming tax credits but you have up to 3 months after the baby is born to claim so get your claim in quickly & you may be entitled then.
Edit: I have just looked on my award notice for last year & the basic family element is £547.50, then on top of this you have child tax credit element, which is £2,300 per child, this is then reduced depending on how much you earn, so say someone with 3 children would have a child tax credit element of £6,900, but if they earned £20,000 a year they would recieve a higher percentage of the £6,900 than someone who earned £30,000 a year who also had 3 children. So the person earning £30,000 may only receieve the family element (547.50) as their earnings may reduce the £6,900 to nothing, these aren't exact figures, just an idea of the way they work it out. So if you look at your tax credit award notice, at the end of 'Part 2' there is a sum showing the 'basic' family element of £547.50, then the Total child tax credit elements, then the reduction due to your income, then under this the total amount payable for the year, if this amount is more than £547.50 then you will be entitled to the maternity grant. But only if you reach 29 weeks before April x