Just got this by email:

Dear Mr thuc,

Thank you for making a booking in the January Sale.

In return, we're delighted to offer you 15% off your next British Airways flight, anywhere in the world.

So whether you're getting away for adventure or relaxation, you can look forward to another trip for less.

To enjoy your 15% discount, all you have to do is enter your unique code (shown on the voucher opposite) when you book online at ba.com

The hilarious thing is I haven't booked a revenue ticket with BA in 12 months, my January booking was a redemption!