You have answered a lot of your questions yourself...
While you may have an agreement with your landlod for the utility bills, there is no guarentee he has conveyed this to the supplier, and infact he hasn't evidence being the delivery of bills addressed to "The Occupier". Your landlord has pocketed the money, failed to notify the supplier that he is responsible for the bills.
They have undoubtedly sent other requests for infomation to the property which, given the infoamtion you have provided has gone astray. your account has since been passed to a debt Collection Agency who will have access to a wider range of tools, utilities and services to track down the occupant of the property, namely you.
As no prior agreement exsists your supplier and debt collection agency are well within their rights to hold you responsible for the payments for the energy that has been used at the property. The fact yo may have ana greement with another party, as stated previously now makes this a 3rd party dispute which the supplier will more than likely not enter into.
The onus is now on you i'm afraid to say to get contact your landlord and get them to approach the supplier in question, and make the arrangements for the bill to be settled. If this fails taking them to a small claims court as previously suggested might be the best avenue as they have the scope to look at the paperwork you hold and rule in your favour.
If you take the latter option it may be worth contacting the DCA or supplier and see if they will accept token paments until the court date, on the basis that if the ruling is in your favour then these payments are returned and full payment sought form the landlord. The basis being that is harder to take someone to court for non-payment if they are makng payments, albeit token payments.
Contacting your local CAB is also something you should put very high on your to do list, the infomation, advice and support they can offer will be invaluable during the resolution of this problem.
As a side note: £8000.00 debt on a commercial property is small fry for your supplier (obviously not for yourself), you'd be quite low down the list of repayments they would be looking to reclaim. It is not uncommon to see 6 figure balances on accounts, which quite obviously are of a higher importance for a supplier.
The answers I give you may not want to hear. The answers I give may not be easy. The answers I give will be the truth. If your prepared for this - ask away!