NX Rewords, National Express review: I want my £15 back

I have been charged £15. I have never subscribed to it. I just use the National Express app to buy tickets. I do not have any account in their service. I could not login on their website to check my status. I wrote to them and they stop this programme but I want this amount back. I asked them for any records of this transaction and they informed me to contact National Express. When I contacted National Express I was informed to contact NX Rewards. No one wants to give me any details.