After much rummaging I have found the Traders Road Transport Association (TRTA) 1965 Yearbook. This contains an advertisement from BTC for their ‘Four-In-Line’ trailers claiming 10% extra MPG and extended tyre life “resulting from the lowest tractive resistance (28lbs per ton)”.
This would be due to the each wheel being able to rotate at its own speed rather than scrubbing itself against the road while being dragged by its twin.
Somewhere on here is a very small picture of a Percy Henley Mercury unit pulling a 4 in line trailer loaded with hops.