The customers don't sound like the best, but look at it from their point of view and imagine being in their shoes.
There are problems from day one on the job,
The heating then doesn't work and someone else has to come along.
Then after the install is supposedly finished, they get told its been fitted wrong and it needs altering.
I am also guessing it has taken longer than they expected due to these circumstances.
Now this is not meant to be a pop at PJF at all, but maybe it will help it look at it from a different perspective and then help him with dealing with the customers.
If this happened to me in my house, I would assume the tradesman was incompetent (not saying you are, you seem to know what you are talking about from what I have seen on here) and I would most definitely not being paying in full. I would also be looking to get the work checked over and any alterations done by someone else and deducted from the bill for the original install.
Now this may be difficult but you need to grovel and offer to put right at your expense and then offer a discount on the job for the inconvenience caused, this is what I would do. We all have nightmares at work and make mistakes which ultimately cost us money, be it an incorrect diagnosis of a boiler fault, having a leak or managing to damage the customers property. It is how you deal with the customer and situation that counts.
