FWIW, I used to get this all the time. In the end I set up a contract and issued it to every job over £300. In it it specified that if it wasn't detailed on the quote then it DID NOT get done. I explained that it was to protect them and to manage their expectations.
The contract also contained really onerous clauses re non payment too. Basically it said if there was no good reason for non payment I could kick the door down and take goods to the value of my debt and had only to make it secure afterwards. The thing was, that those clauses were only of interest to those who would try to rip you off.
Probably illegal, but in all the jobs I had only three people questioned it. I turned those jobs down.
By detailing what you do, you can always refer to the contract. In addition I always put in a paragraph which said if there is ANYTHING you either do not understand, are not completely sure of or have any concern about THEN SEEK CLARIFICATION before agreeing or signing. Oh it also said you can't cancel with out penalty.
Point is, using it you'll soon find out who the robbing swines are. Then you can decide to walk away unscathed. When produced in court, the court stays on your side as you've tried to do what you can BEFORE the event.
Happy to share if it would help.