According to MoneySavingExpert, select Amazon UK accounts are being targeted for an offer whereby you receive a £10 amazon voucher in return for buying £40 of amazon gift cards.
This is apparently the link to check if your account qualifies:
If successful you are told what to do on the screen.
If like me you are unsuccessful then it says:
Unfortunately, you are not eligible for this offer.
It seems to be 25 to 50% success rate, so worth trying. No idea if there is a quota to fill before it runs out.