Thanks for you input Gary,
Your thoughts agree with my understanding of LPG bottle filling in the UK, however bottles that comply with UK regulations also carry an over pressure relief valve that will begin to vent excess pressure. The professional view is it's better to have a controlled vent of vapour that can then burn off, rather than a burst. Also the venting of the vapour will cause the bottle to drop its pressure (latent heat of vapourisation) which actually reduces the danger (Though I agree with the Fire Brigade - keep well away and dowse with lots of cold water)IN extreme cases the brass valve will melt (and shoot of like cannon ball) and allow the content to vent. Bottles must be secured in an upright position to ensure that the valve is in the vapour space above the LPG. The fire brigade will evacuate who areas if LPG tanks are not secured or are observed to be on their side.
I quite agree that privately owned refillable bottles could present a danger if over filled by their owners, but in doing so they automatically invalidate their insurance and endanger themselves. It is also possible that some refillable bottles acquired abroad or from some locations in the UK may not meet the UK standards for construction - some do not have over pressure relief valves, and some are made from plastics which may have sufficient strength when cold but become pliable and more likely to rupture at only modest elevated temperatures.
As to the strength of the different sizes of steel bottle, I am not aware of a different standard of construction, all should meet and exceed the test pressure which will be significantly greater than pressure relief valve setting, but by simple physics, the greater surface area of a larger tank means that the pressure inside will have greater potential energy in a large tank compared to a smaller one.
PS - ES have still not replied!