Companies are not allowed to create a financial link known as an 'association' based on family relationships or address. In other words, if you live with a partner who has a poor credit rating then this cannot in itself affect your credit rating.
However, an 'association' can be applied for if you have joint financial commitments such as a joint account or mortgage together. It can take a while for companies to update their credit ratings so if you sever the financial ties you should arrange with any lenders or utility companies to update their records as soon as possible.
Ensure that any companies who are still writing to your former partner via your address are informed. You can also ask credit companies to create a "dissociation" on their records.
If you are considering buying your ex out of the mortgage see our section on Buying a Home. If you cannot break all of the ties because, for example, you have agreed to continue being jointly responsible for the mortgage, then you can still advise credit companies of the situation and ask them to put a note on their files accordingly. Please note that child support and maintenance are not considered by companies as joint financial ties.