Yes 6 shillings and 8 pence, my conversion was wrong. I normally put in 50p worth at a time in those days when the £2 only rule was not in force.
One time I remember queuing for my £2 worth, there was a huge Bentley behind me. We had to queue for at least 10 mins, when the Bentley driver tried to put his £2 worth (6 gallons) he only got 1 gallon in before the tank overflowed. So that gallon cost him £2 in 1973 !!!. His tank must have been full to the brim when he started to queue , serves him right for being greedy.