Speaking as a homeowner (with experience as a supplier), if I received the info provided by SaveH2O, I would: -
1) Discuss the problem with the builder, it's his problem to meet standards, not just scratch his head. Now they know the problem is also at the boundary and the adjacent property they know they must look upstream.
2) If the builder fails to follow-up and accept responsibility, you need to go to your local tribunal, or have your solicitor write a letter.
Ed