What meldrewreborn said!
Let me illustrate a f'rexample.
My D D is £40 due on the 1st of October for my September usage. I pay balance in full every month.
So, £40 of my money is taken by Eon.next. No huhu...without the grant, that's what I would have paid anyway.
03 October, the gumment credit my account with £67, putting me £67 in credit. Cool beans.
Prices went up, so my bill for October will now be about £50. Fair enough.
Only, my account is in credit so instead of taking the DD, they take my payment from my account credit, leaving £17 in the pot.
Again, the gumment come along sometime in the first week of November, 4th say, and deposit another £67.
So far, all I have paid is my £40 for my September bill, which was pre handouts.
My account is now £84 in credit by the time my November bill of £80 rolls in. Dark evenings and getting colder so I'm using more.
No need to take an £80 DD because my account was in credit, see?
Along comes my December £66 on, say, 2nd December. I'm now £70 in credit, but get a bill for £95 because I cooked loads of dinners over the period.
So, on Jan 1st, they take the £70 credit from my account, and claim the balance of £25 by direct debit.
Jan 4th, another £66....and, well you get the picture.
Over the 6 month period, you will have actually shelled out £400 less than you were billed for.
Your direct debit payments are not affected in October because that D D was for your September bill. Direct Debits are paid in arrears, the grant payments are an advance. You will see the effect on your DD only from your November payment, which is for energy used in October, which is when the scheme came into effect.
