My rate is regarded as high by some consumers the majority appear to agree they get value for money as most of my business is referrals also the consumer is advised of the cost before they commit to placing an order be it written or as accepted by over 90% verbal agreement.
I have a few issues with this:
The valve used should be deemed as standard stock in most plumbers vans, if it's not in stock then don't be charging your full hourly rate for going to the nearest retailer and buying it, unusual items that need a van to collect for example a boiler or oil tank means the customer is getting full value for the use of your van and your time.
By all means have a mark up on your van stock, it costs money to put it there and extra fuel to transport it regardless of whether it's needed for a specific job or not.
I would have given the customer a guide cost (estimate) it works for both parties as the customer knows how much they are spending, if they can afford it, if they believe they are getting value for money, maybe look for another estimate or just say no, the contractor knows he can buy the custom part or take it from his van stock knowing he will be paid for the job.
In the case described we are dealing with an elderly person, personally I am lucky enough to be able to do the job described and promise an invoice that is very often never sent or depending on the customer charge a nominal amount leaving the customer happy that they had a job done and it's paid for, each customer / job is handled in a different way because the customers like ourselves are individuals.
If we round up numbers could we go to £5.00 for the valve?
Maybe an hour allowing for traveling to and from the job?
Even in the days when money was in extremely short supply (yes I've seen those) I couldn't justify charging any more even if I had to leave to buy the valve.
Regardless of our beliefs we all know (or should know) right from wrong, a couple of very rough quotes to mind:
Do unto others as you would have done unto you.
Whatever you do unto the least of my brethren you do unto me.
We can call it Karma, natural justice, payback, whatever comes to mind but I have no doubt that man will pay for taking advantage of the elderly.
Maybe he is already paying because it appears he forgot that he left a detailed description of the works but had to lie when asked what work he had carried out for the amount charged?
Perhaps he didn't break the law of the land (I'm almost sure he didn't), what he did was enough to make a lot of us (well me anyway) feel sick that he would dare to call himself a tradesman.
