Had a similar issue with Scottish Power in March.
The previous occupier moved out in October 2020 - so a good 5 months.
Scottish power kept sending me a bill, then an amended bill - which each time was considerably more than the last one.
From day one, I was with Octopus Energy and had done all the necessary admin to ensure successful switch to Octupus on the day I moved in.
Octopus uncapped the gas and installed a smart meter on the same day.
Scottish Power [previous occupants supplier] insisted I was with them and refused to acknowledge I had formally switched - a process I began in January 2020.
They insisted I owed them 300 pounds - in spite of the fact the gas meter had been capped from October to March.
Like the posters above, I spent days phoning - waiting - being cut off - phoning...when I did get through, the person at the other end just read from a screen, details based on my postcode and house number. It was like being talked at by a robot.
I explained that the property had been vacant since October 2020 - but she insisted I had to pay the bill!
My solicitor [at a cost of 20 pounds] sent them a snotty letter:
I received a "Final Demand" for 495 pounds!
In the end, it took them until June this year to admit they had made a mistake and sent me an apology letter!
Nightmare.