If you have a joint product with someone else, then their credit history will affect yours when you apply for any future credit.
This is because having joint products financially links you with the other person. If they have a good credit history, then there's no problem, but if they have a bad credit history, then you could find yourself being turned down for products, even if your credit score's tip-top.
It's worth noting that some utility bills will link you to old flatmates. Check your Experian Credit Report within Credit Club to see if you're financially linked, and if it's an old link, use the ‘Need more help’ support to contact Experian and ask it to delete this link. You may also need to delink yourself with the other credit reference agencies (Equifax and Callcredit) as the link may be on other credit files too. You can click on the “Help and guides” section in Credit Club, and then on “Check other credit reports” to find out how to check if this link shows up on your other reports.