I have viewed their sites, and I have found the PureLPG comment
Here
They say :-
PureLPG say:-
"Butane is a naturally flammable gas obtained from oil refining and natural gas processing. It has a boiling point of 0°C which is right around the same temperature for freezing water."
The word "freezing" relates to water not Butane.
PureLPG say:-
"Propane burns hotter than butane due to its chemical structure, propane – C3H8, Butane C4 H10. Propane has a higher calorific value which means it gives out more energy than butane. Propane has a hotter flame temperature, hence why propane is the preffered fuel for cooking / catering applications (as well as heating)."
Technically correct Propane 1980C vs Butane 1970C, but the difference is so small the man in the street wouldn't be able to tell the difference.
There are several differences between the gasses, which makes direct comparisons technically difficult. The Industry use a special calculation to generate a WOBBE INDEX value for each gas, which indicates the the relative compatibility of different gasses with a view to using them in an appliance.
Broadly speaking Butane at 28mBar and Propane at 37mBar supply pressure will generate the same heating effect. At 30mBar Butane will will provide more heating effect than Propane.