So why leave it on a one pipe? , surely it should of been converted to a two pipe considering the pipework required replacing.
Indeed. I was thinking that.
It is not unfair, his first post said "I'm concerned about the longevity of this installation".
That is all that matters, he is the paying client, if he is not happy with the work then as the client he has the right to ask for it to be changed, it is pointless asking on here, you could show the picture of a very neat installation and I bet someone on the web would find fault somewhere, you are only looking at pictures, this does not always give the full outlook, but if the OP is not happy then he needs to say he is not happy and he needs to say he will not be paying until it is rectified.
Sorry, but you are still missing the point completely.
If the guy doesn't know about plumbing then he can't assess this himself.
If everyone here had said something along the lines of "not as tidy as it could have been, but it'll be fine 'forever'" then that may have dealt with his concerns.
How many times do you hear of trades having done a job, but the customer won't pay because they are unhappy about something irrelevant. Had it been an "OK" job then coming and asking here would have avoided that awkward situation.
As it is, people have pointed why he is right to be concerned - which means he can now discuss the matter without (potentially) going into battle unarmed.
We hear plenty of stories where the customer is unhappy, but ends up paying because he doesn't have anything to back up his concerns and so if "out gunned" because those doing the bad work can speak with authority - you know, "been doin' this for years ...". What if they start talking about the pipes and fittings being way over spec for the job, "they'll take 20 dynamic megaflugels, but this system only runs at 5 flizens per hour" - complete b***ocks, but someone who knows nothing about plumbing wouldn't have a chance of countering that.
Now, if it were to "get legal" then he'd need to get someone else in to give him a proper report on what's been done wrong. As you suggest, asking "some blokes on the internet" won't do for that.