A typical plumber in London has seen all types of residential customers. From the very poor who count every penny to the very rich who want the best available kit (and price is no object). From customers that are nice and friendly, who make your day more manageable to customers that feel like they a point to prove, and the plumber is their target for the day.
There is no one single magic formula on how to deal with customers. However, I believe there are several factors that are always relevant.
Be honest with the customer. Even if the plumber is facing uncertainty (as many times happens when the plumber cannot diagnose the problem or find the source of the leak), it is better to be open and honest with the customer. The customer will respect your honesty and will credit you for that. Be professional. Execute the job to the highest standard. Not just to the regulatory required level, but actually to the best level you can possibly reach. Put yourself in the customer's shoes and try to exceed their expectations. It is difficult to argue with a job well done.
Be fair. Use your common sense and your commercial experience. Sometimes giving up on a small amount of money (e.g. when there is a dispute over parts of the job), can actually result in a happy customer that respects your judgement, even if there were problems in the installations (some problems are unavoidable). Such a happy customer will lead to repeat business. Isn't it what we are all looking for?
Be friendly. I know that some customers can be difficult sometimes, but as long as you try to be friendly and helpful with them, this can help diffuse a tense environment and help create a more pleasant situation. Remember that many times the customer is in a state of crisis and they can be stressed. Being friendly and helpful will reduce the stress level for them and subsequently for you too.