Old supplier chasing when they never were my supplier

I moved into a property (rental) on 26.10.18. I set up my account for BULB immediately and used the meter readings as they were on the day that I moved in as my opening meter readings. I then provided BULB with the readings as necessary since.

Co-op energy are now in touch demanding that I pay for energy used between 26.10.18 and 25.11.18 as “they were the supplier at that time”. I have no account with them or ever have had an account with them. I assume that they were the supplier for the previous tenant / landlord- all readings would be estimated as I have no idea what the readings were for the period that they are claiming for (as at 25.11.18).

I have paid for all the energy that I have used during my time of occupancy but they have continued to chase it. Is there anything that BULB can do for this as I feel like I’m the one stuck in the middle of it all being asked to pay for the same energy twice.

I had no idea that they even were the old supplier.

When you move into a new house, you need to sign up with whoever is supplying gas/electricity to that property at the time you move in.

"When you’ve moved into your new home, you should:

contact the current supplier at your new property to tell them you've moved in - you can find out who to contact if you’re not sure who your supplier is or if your property doesn’t have a gas or electricity connection.
read the meters on the day you move in and give the readings to the current supplier, to make sure you get an accurate first bill - you’re responsible for the bills on the day you take ownership or responsibility for the property, even if you don’t move in on that day
pay your old supplier’s final bill when you get it

You’ll automatically be put onto a ‘deemed contract’ with the current supplier of the property. This will normally be one of the most expensive tariffs, so you should look for a better deal with the current supplier or a new one as soon as you move in.

You can only change suppliers from the day you become responsible for the property. Switching will normally take about 21 days, so you’ll have to pay at least one bill with the current supplier."