Hello, Pigale!
It is simply a division between two houses! There must be houses all over England standing next to each other like this?
We live in a semi-detached house, but we are attached to the house on our other side.... the passage way (for want of a better word) lies between us and the other set of semis on our other side.
It has never proved a problem before.
The land belongs to the houseowners (yes, it would half and half)but the access is considered communal as it is a shared access to the back gardens.
Thames Water is making it a technical issue!
It would not have been a problem had the new owner moved in.....but no- one has a contact number for him!
However, TW is contacting the Land Registry to obtain his details.
Hopefully, progress will be made soon!