I think they got their fuel prices confused at the Tesco petrol station in Scunthorpe, when I was there last week. Diesel is never cheaper than unleaded petrol.... until now, that is. Me thinks someone at Tesco made a mistake.
That's the 'per litre' price, so the diesel was about £4.40 per US gallon.
I double checked the prices shown on the bowser, before I filled up the Toy. Yes, the diesel was 4p cheaper per litre.