A statement of claim is more than a letter, but you could start with a letter of demand. Probably would be charged by the hour say $250 for the letter and then ay phone calls etc on top.
K could always begin the process by issuing a statement of claim. The otherwise may get a lawyer and fight it or they may just admit liability and/or make an offer. Then again, they may not!
K what about your insurance?
Not much you can do if you don't know who caused the damage. If it was a plumber then the employer would be liable, but you seem to have little evidence for this at this stage.