I have a huge problem with the idea (often repeated here) that a failure to contain water in one person’s apartment is not their responsibility if it accidentally floods another unit – and that the affected owner has to prove the upstairs owner’s negligence.
I would think you might well start from the indisputable fact that water went from one apartment to another and then let the person who owns the flat where the water started its journey prove, if they can, that they were not responsible. Since this started with newly installed equipment, I’m guessing there’s a fair chance it was the installation that’s at fault.
The link to the article above doesn’t help much. I would be interested to see a Tribunal ruling (if one exists) that the affected neighbour had no claim against the upstairs owner unless they could prove negligence. It’s a lot harder to prove someone didn’t do something than it is to prove they did.
So – until someone shows me a tribunal ruling to the contrary – I am going to agree with stratalawyer (above) that there is liability and, if it was caused by a poorly installed fitting, then the upstairs neighbour should chase their plumber for the costs.