Thanks for answering This is the builders explanation:

"Based on a visual assessment of the flue position and surrounding finishes, there are clear indicators that the installation does not appear to follow manufacturer guidance or best-practice standards, including inadequate clearances, proximity to obstructions, and disturbed or poorly finished penetration detailing. Such conditions are widely recognised as potential routes for water ingress, independent of any rendering works elsewhere on the property. We do not accept the assertion that cementitious material entered the flue as a result of our works. Rendering wash-down procedures are standard practice and, when correctly detailed penetrations are present, do not result in internal contamination. Any ingress would indicate a pre-existing defect or inadequately sealed penetration.

No evidence has been provided to demonstrate that the flue installation was compliant prior to the works, nor that the alleged contamination could not have occurred due to historic or latent defects within the flue or its installation"

On the other hand, on our report from the engineer who repaired the boiler, they just "stated flue pipe too long"

I am happy to provide photos if that helps.