As I said- it depends on your supplier.
The participating ones only have to give this discount to the 'core group' (those getting the guarantee credit element of Pension Credit.) As I understand it, it is then entirely up to them whether to make this discount available to anyone else - the so-called 'broader group' - and what criteria to set. The two who payed it to my elderly mum were First Utility and British Gas, whereas Eon had different criteria, that ruled her out.
But the criteria can change from year to year, too.
If you are over a certain age, don't get the relevant Pension Credit, but have an income under £1600, it is worth inquiring about this when choosing a new supplier.
Similarly, even if you do get Pension Credit and already get this, check whether a potential new supplier is part of the scheme before changing, or at least see if losing the payment still makes the move worth it!