It’s a common myth that people who live at the same address as you, or even the previous occupants, affect your credit rating. But don’t panic, the only people who can impact your credit rating are yourself, and anyone you have a financial relationship with i.e. someone you have a joint account or a loan or mortgage with.
Lenders will check the scores of anyone you’re financially linked to as well and may decide that their circumstances affect your ability to pay them back. So even if you only have a joint account with your flatmate for your rent, their unpaid credit card could have a negative impact on you. If you do have any joint accounts with someone you are no longer financially linked to, make sure you consider getting removed from the accounts and if you do then write to the credit reference agencies asking to be disassociated.