Any bottle filled by a company not duly authorised by the refining company the cylinders belong to, is considered to be an illegal filler.
When purchasing refills from illegal/rogue fillers, there is no guarantee of the actual quantity of LP gas fill, this is because some fillers are known to partially fill the bottles with water to create the illusion of a full bottle. This is also why they are often cheaper. Water inside the cylinder is also dangerous as the it can cause rust, which weakens the cylinder.
Legal Fillers/Distributors (ie GotGas) Sell bottles that are regularly pressure tested, and carry a Public liability on all cylinders they fill. This public liability is instantly void when they are not filled by an authorised filler.
Illegally filled bottles are easy to spot:
1. They have clear unprinted seals over the cylinder valve, Legitimate fillers always have printed seals eg TotalGaz, Afrox, Bp Gas, Easygas.
2. One gets a mix of cylinders from the illegal fillers, ie some Easygas, some Afrox, some Totalgaz etc. Or a different one everytime if ordering singles. GotGas will only supply you with Orange Totalgaz cylinders.