Neighbours have blocked off drain

We live in a village with neighbours on two sides. These are traditionally built houses. Our courtyard drainage runs under the neighbours property into the main sewer.

After the house was purchased recently the new owner has demolished the property and is rebuilding it. The drain did not seem to be a problem for them to start with but they have now told us that they intend to block the drain off.
This will flood our courtyard and house and potentially affect other neighbours as well.

Can they just do this? Any advice would be appreciated.