False information

I just filled my car by a shell garage. There is an advert outside that says if u fill more than 50L of diesel you get a coke. I filled my car with 54L of diesel and paied via my garage. And and swiped my clicks card as clicks and shell has a running promotion. When I asked for my coke I was told by the attendant that I need to fill more than 100L of diesel to get a 2L coke and if I put anything less I get a 330ml coke but I forfeit getting a coke altogether if I swipe my clicks card as I can take advantage of both offers at the same time. There is no small print on the board. The board also says nothong about t's & c's apply and the attendant says he was supposed to tell me when I wanted to pay. When I called the shell complaints line I was told the garage just pays for the brand name of shell and further to that they have no policies, etc regarding personal offers and they can do what they want. Surely this is not right?