If the cylinders (presumably metal) are in contact with the ground outside and the LPG is piped into the house via a metallic pipe then there is a possibility that an earth potential different to that at the MET can be introduced onto the hob, therefore in that instance I think the pipe should be treated as an incoming service and bonded to the MET. However, as Andy suggested, if the pipe is not metal, ie extraneous, then there is no need to do anything.