10 years is a long time for a company to come chasing, i would guess this has been sold on to a debt agency and they are misleading you to who they are.
I would ask for all the paperwork before continuing communication with these people - in theory they could be anyone.
Is there some way you can contact your ex? I understand that they can come to you for the full amount even if it is joint names so it would be in your interest to find your ex.
A debt never really goes away, it can be sold on and can still legally be chased.