Price of Diesel advertised and confirmed as R12.48 p/L but charged R12.89

I always refuel at Engen and found that prices for Diesel 50ppm was different at the various branches, so with this I always go for the best possible price. At first I used to fill up at the Groenkloof station until I realised this price difference. Yesterday, my fuel went on low and had passed Groenkloof in the morning with R12.48 advertised but in my mind their price was higher through experience so i went to Sunnyside during lunch only to see that there it was R12.79 . I decided not to refill there and went to Groenkloof after work since it was R12.48. I was assisted by a friendly attendant by the name of Thabang on pump no 2, that I confirmed with that it was indeed R12.48 before i filled up. To my surprise upon paying, I was actually charged R12.89. I asked Thabang about this and he said that they probably forgot to change the price at the garage entrance from R12.48. I feel ****** and that we are being conned into choosing to fill up at this garage thinking we won't notice these prices. A 41c price difference is a lot and adds up to a total of R18.50 for the 45.145L I filled yesterday. This is unacceptable. I am very disappointed and demand a refund for the difference.