In 2013, the government announced it would cut green levies from bills in order to save consumers money off their escalating energy rates. This freed up cash for all households in the UK, with £24 off of everyone's energy bills — split into two instalments.
The first half of the £24 was refunded to households in the winter of 2014/15, and this second instalment of £12 was automatically given to those on standard meters in October 2015. Prepayment customers may have to reclaim their rebate manually, but only have until 29th February to do so.
With prepayment customers often getting a raw deal on the rates of their gas and electricity, this £12 free credit should be snapped up before the end of the month.
If you were a prepayment customer in October, you should have received a letter from your supplier about the £12 credit you're entitled to. This will be in the form of a voucher, or alternatively will give details on how to claim it.
However, if you think this might have ended up in the bin or you never received the letter between October and November 2015 when you should have, you need to contact your supplier to let them know you should be entitled to it.