Ok so I think I have figured what's going on, the latest refund for free fuel happened too, £85 
In the Esso app you set a fixed amount, for me me I have it at £100 and then the app/system refunds the difference back, so if my fillup was £85, then £15 gets refunded within a day or so. What seems to be happening is instead of refunding the £15 difference, it's refunding the opposite and I'm getting the amount I filled up for refunded.
Not bad.

In the Esso app you set a fixed amount, for me me I have it at £100 and then the app/system refunds the difference back, so if my fillup was £85, then £15 gets refunded within a day or so. What seems to be happening is instead of refunding the £15 difference, it's refunding the opposite and I'm getting the amount I filled up for refunded.
Not bad.