The following article was published after petrol had hit the very high price of 6 shillings and 8d (Old pennies)  per UK gallon = 33p per gallon or £0.0726p per Lt . The cost of LPG at this time was 2 shillings 10d (Old pennies)  per UK Gallon = 0.15p or 0.033p per Lt. The cost of a  LPG conversion in 1970 was £135.00.

Why did it not catch on? well at that time there were very few filling stations 20 or so, the tank took up all the boot, and the engines were not designed to burn unleaded fuels, unlike the cars of today, so valve recession and pitting damaged the engines. If they had flashlub at that time it would have been diferent. 

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The article preceded the fuel crisis of 1973 when petrol was almost rationed, in fact many motorists received ration books, as the UK prepared for the worst, but thankfully the rationing was never activated.

This article explains the early system very well, but would not be excepted now under the LPGA codes of practice. (please see the man filling his tank)