I read this post from Calor on another caravan forum;
Just to confirm from our end, an end user of cylinder gas will always be charged 5% VAT for the gas.
The first time a customer acquires Calor’s cylinder gas, they enter into a cylinder refill agreement which is a one-off charge that means that when an empty cylinder is returned, they are immediately entitled to a replacement cylinder filled with LPG, and they only have to pay for the gas (at 5%) at each subsequent cylinder exchange.
Due to a HMRC ruling, if the gas is purchased directly by Calor, the refill agreement carries 5% VAT, but when the gas is supplied by a Calor dealer, then the refill agreement carries 20% VAT. However, the price a customer pays for this refill agreement will always be the same (currently £34.25 including VAT) regardless of where the gas is purchased.