Why can't you show a copy of the agreement?
That is a problem I'm afraid.
There is always a hierarchy of liable persons with utilities bills and the owner of the property is at the top. They will pursue another person if you can produce a tenancy agreement but you cannot here.
Therefore you are likely to be considered the liable person.
You could recover by trying to sue the tenant if they can be found. Obviously you can only do that if your agreement was that they would pay.
Can I clarify anything for you?
You are entitled to know how this was accrued.