Response from South Downs Tile & Stone
I tiled both the existing floor and the new extension area (where screed and underfloor heating were pre-existing conditions not installed myself). The issue was isolated to the new area only.
The customer acknowledged the issue was not my responsibility, agreed to my proposed solution, and even offered additional payment to rectify the situation the second time around.
We have messages on file to prove this conversation took place.
We visited the property and replaced four tiles for free.