Jesus, what a dog’s dinner. That’s why plumbers are supposed to wipe the excess flux off joints after they’ve cooled down. Flux left on will collect moisture and slowly corrode the pipe… as you’ve found….
That’s going to be an absolute barsteward of a job, to be honest. If it was my house I would take the whole mess out and re-make every joint on the basis that if one has started leaking, the others won’t be far behind.
The previous owner had some right dodgy tradesman in, another example of where she went on price rather than quality.
And as with what globalti has said, get him to remove the blanked off end and just do a tidy right angle (unless you think the blanked off bits are defo needed in the future).
I’ve only just noticed that, what a poor effort to leave that whole section on. Like you say, easier to take it off and get right angled piece put in.
Thanks all, plumber it is then.