Credits from weekly offers such as “A little help with your heating” are credited a month at a time - in the month after they were earned.
See the offer Terms: https://www.ovoenergy.com/terms/beyond-terms-and-conditions
8. Bill credit process
8.1 For Gas/Dual Fuel customers, gas used in the free heating hours selected will be calculated and paid back as a bill credit. Your bill credit will normally appear on your account in the following month, for example November will be paid in December, although it could occasionally take longer to credit your account.
8.2 For Electricity only customers, you will automatically be allocated the 10 kWk cap amount and this will be paid back as a bill credit.
8.3 This will appear as a separate line item on your energy bill, as a reduction to your price payable for energy.