BP - False advertising of a free Coke for R1000 diesel - no alternative drink exchange allowed

BP Halfway Gardens, Janadel Ave, Midrand advertises a free 2liter Coke when filling up with at least 50liter diesel. When you claim the 2liter Coke, and want the sugar free version for health reasons, they refuse and say that only the normal coke (R33) is on the system for exchange of a R1000 diesel payment.