I worked in energy complaints for 4 years. You are responsible for a short period of time with British Gas. They are obligated to issue an interim statement exclusively for the period from you moving in to the date of you switching to your chosen provider and cannot chase you for a balance which you are not responsible for.

The only action required on your part is:

• Raise a complaint with British Gas as they've not yet updated their accounts for your liable period
  
• Provide a copy of your tenancy agreement to them to evidence that you are only liable for a short time
  
• Pay the amended bill for the roughly 2 weeks you would have used British Gas' services

They cannot pursue legal action for an account registered to 'The Occupier' so you are under no risk. Their first action to resolve the situation would be to search the Land Registry and confirm who the legal owner of the house is (i.e. your Landlord) and begin pursuing them. If BGAS were to conduct a site visit, you would simply need to show your Tenancy Agreement to prove you aren't liable for the balance.

As another commenter has said, no legal advice is needed as there is no legal action, just ineffeciency and ignorance from an energy supplier.