Water company to investigate/ rule out any main or sewer supply.
Buildings insurance to cover the cost of reinstatement if covered.
If water main leak it will also be covered under trace & access part of the policy, usually around £5k cover on a domestic policy.
This was my next thought.
https://fixmyplumbing.org/plumbing-problems/coil-in-hot-water-cylinder-split/#:~:text=It's%20quite%20rare%20but%20the,boiler%20pressure%20will%20rise%20continuously.
If you’ve got gravity fed cold supply to your toilets and you think it may be them, the best way to check to see if they are overflowing internally is put a piece of toilet paper on the back of the pan just above the water line. If that gets wet, then that’s your leak.
If your cold’s gravity...
Very much this ☝️so get the ball valve changed first.
Depending on age of house, sometimes you will have hot/ cold/ heating pipes under the screed downstairs and as a water leak detective I’ve seen this many times where the pipes can pinhole, so if this has happened on hot supply it will let...