Paper trail is what you depend on if things go to court, so you should have quotes, invoices, letters (texts, emails or in paper). If the customer makes a complaint - like this alleged leaking toilet, be sure to politely reply to them and have proof of that, plus proof you asked to look at it and any repair work you might do later. Reason why I am saying that is the courts will find a tradesman guilty if he has been accused of leaving bad workmanship and has not produced proof of what he did to try to resolve the alleged fault.
In Small Claims courts, as far as I know, if you win and the other person refuses to pay, then you can take it further and get the money from their wages, or other property, - that's if your customer has wages or assets.
I always understood a cheque is a promise to pay and anyone withdrawing a cheque would have to have a very good reason to be allowed to not pay, - such as fraud.