Who is responsible for correcting a misconnection?

You, as the owner of the property, are responsible for corrective work. If you rent the property then you should contact the landlord who may be the local council, housing association or a private landlord.

Please also advise  your water and sewerage company to stop any further letters being sent to you.

If you own your property, you will be responsible, even if the misconnection was made by a previous owner. You could try to contact the previous owner to see if they would be willing to accept responsibility.

The survey undertaken at the time of your purchase would not have identified this problem unless you specifically asked for a full drainage survey.