Mess up with energy supplier and getting debt collector letters

Hi, looking for some advice here please!
I started receiving letters from Octopus Energy addressed to The Occupier in August, I assumed they were promotional and didn’t open them. Big mistake!
I decided to open one from AIC debt collectors (again, addressed to The Occupier) and was stunned to see they are chasing a debt of £250!
So far I’ve called the debt collectors, Octopus and also informed the man who came to read the meter that the serial number on the meter didn’t match what he had on file.
I contacted Octopus by email, so far little meaningful response apart from ‘Oh dear, this is a muddle’ which TBH infuriates me.
They both have the same MPAN number for me on file.
EDF have correct meter number, Octopus Energy don’t
EDF say that I changed my electricity supply in April 2018. I absolutely did not and don’t recall any letters from them about it
EDF checked a register and both meter numbers, mine and whoever else is dragged in to this are in fact supplied by Octopus.
Octopus say that on a national register my meter number is registered to another flat in my building.
EDF have successfully been billing me with my meter number since 2014!
Help! How should I tackle this? Any advice much appreciated. And yes, I should have noticed a drop in my Direct Debits, I’ve not been very financially savvy in the past and was off work for a year seriously ill so checking my energy supplier was the last thing on my mind