[Closed] Anyone in the Domestic Energy industry or understand how gas supply works?

A friend of mine lives in a house that has a gas supply but no registered gas supplier. He or she called the gas enquiry number and they confirmed that there is no registered gas supplier. Now say for arguments sake they had been using this 'free' gas for a while and phoned one of the usual suspects (ie Npower) to start using them as the official supplier, how would the billing work? Would my friend give them a meter reading and it goes from there? What if the house has never had a registered gas supplier before this point?

Anyone know any more about this than me?

Edit : Sorry this should be in the 'off topic category'

Any help much appreciated