So I took one for the team and tested the offer. I was in central London for some work so I thought it was best to try redeeming the free sub there, than my local sub which is in a small parish.
My experience was a bit mixed but the end result can be seen in the attached picture. At first, I was told off for demanding a free Sub by showing the app to the server. Luckily, the supervisor who was just starting his shift overheard the conversation and asked me to show him the offer. He scanned my Sub Card and after some fervent taps on the screen, he confirmed there was a free Sub available to redeem. He asked me what toppings I want and gave me a free Sub.
Long story short, I won’t blame Subway for not honouring the free Sub offers. But as my experience clearly shows, it is down to the person on the till to find the free Sub voucher on your account. And in my experience, not every person on the till is an expert because a lot of servers are students and only work part time at Subway. So be patient and try a different store until you find someone who can crack it for you. I would recommend visiting a Subway with a high footfall and one which is located centrally.
Best of luck to those who tried and got it on their apps. My 2p on the offer is - Don’t give up! (edited)
