I need advice from a Canadian real estate lawyer with respect

We bought the house in October, 2004. In the spring of 2005, we noticed a small leak in the basement. I removed the drywall and noticed a small crack, so I purchased an injection kit and fixed the repair myself. We left the wall opened until the fall of 2015 to make sure it did not leak again, and it never leaked. We fixed the drywall for the sale of our house in the fall of 2015. In the fall of 2015 we sold our house, with a Feb 2016 possession. We declared in the sale that there was a leak discovered in 2005, and that I had repaired it. I also showed the buyers where the leak was and explained to them what I did and the history. I re-explained this to their home inspector when they did a home inspection. They signed off that they were satisfied with the inspection of the house.Unfortunately, for some reason, just after they took possession in February, the leak started again. Now the buyers are angry and aggressive with the real estate agent. The buyers and sellers agent was the same person in this transaction.They want me to fix it, however the repair I did is almost 11 years old. Had they been polite I would perhaps have listened to them, but right away they got very aggressive.Am I responsible to fix this, even though it was 11 years ago, and was fully declared in the sales contract ?

the sale was conducted in the province of Quebec

I should also mention that immediately upon possession of the house, the buyers started an extensive renovation and modification of the house, basically tearing a good portion of the interior of the house down to the studs and plywood floor. The leak was noticed after they did the demolishing of the interior walls, so in my mind there could be some question as to whether their work impacted the structure of the house, but this is just conjecture on my part