Found this at the bottom of the Lightning deals page in the middle (to the left of the "Amazon coins " logo and to the right of the Pantry offer). I didn't receive an email notification of this deal from Amazon (as yet).
Tried it and it works , Bought a £100 Amazon Gift card Top up (by following the link) and £105 credited to my balance .
I have bought Amazon Gift vouchers in the last year ,so not the one that most of us are excluded from(when you have bought vouchers in the last 2 years ) , maybe its the same for others?- worth a look .
Don't know whether it had anything to do with it but earlier I checked out the Prime T&Cs (not interested) and before that used some of my gift balance to purchase an item .