Unauthorised account transfer, illegal or not?

thelicencee

After a relationship breakup, the person responsible for paying the DD for gas and electricity to BG, notified them that they had left the property,  although they were still the freeholder.  They requested that the accounts be changed into the name of a third-party. BG did this without any authorisation from the third-party,  after the third-party complained BG cancelled both accounts,  but have continued to send high tariff bills to the third-party saying that the third-party is automatically liable. 
Is it legal to transfer an advanced credit agreement to a third-party without authorisation,  is the third-party responsible for the bills, which also include numerous extra charges for late or non payment of the disputed bills. BG also gave wrong information with reference to the warm front grant , so this was not credited to the disputed bills.
This grant is due as the third-party is severely disabled and is on the BG prority customer register,  for both gas and electric, and has been for many years. 
Any help woukd be appreciated as would the legal specification with regards to the unauthorised transfer of an advanced credit agreement. 
Many thanks to all those that are able to offer advice. 

MWT

Was the 'third-party' living in the property?

If so then they were jointly liable and could have been billed anyway even without their name on the account...

Ectophile

Is the third party still living in the property?  If so, then they are responsible for paying the bills.  If that's the case, then they should be getting the billing sorted out properly, rather than quibbling about details.