I think it's only calor who charge a deposit on their bottles , if you go to a different gas supplier I'm sure you just pay for the gas , with the bottle being free .
According to the FloGas website "Please Note: A cylinder non-returns charge of £30.62 per item will be added to your order" (for a 6kg propane, larger sizes seem to have a lower charge ).
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So seems best to get Propane if using van all year round and no hassle with regulators, or temp problems, is Propane a lot more than Calor, does either last longer than other one, I have the Red/orangey colour bottles.
Calor is the supplier of the gas, propane or butane is the gas. As advised in the the thread use propane then you are ok for all yr. Price difference butane/propane is much the same, so use propane. If you have red Calor bottles you have propane.