Kitchen sink unit leak

No I don't think so, but for me whatever job I do, most be check properly before living the site to avoid customers calling for same and pay more money. I will check everywhere make sure that all is well done, if there's problem is my duty to go back and fixe it up without paying because I may mistake.

In principle, not automatically.

In the UK, a plumber providing a repair service must carry it out with reasonable care and skill under the Consumer Rights Act 2015. If the service was not done with reasonable care and skill, you can require the trader to redo the work at no extra cost, within a reasonable time and without significant inconvenience.

The practical issue is whether this is:

the same leak / same fault that should have been identified during the original visit, or
a different defect that only became apparent afterwards.

If the further leak was coming from the same under-sink area and should reasonably have been found when the plumber tested the repair, you have a fair argument that it is part of the original job and should be put right without an extra call-out fee. A plumber repairing a leak under a sink would normally be expected to test the system properly before leaving, especially after working on the connector and hot pipe. That said, whether he should definitely have spotted a cracked U-bend depends on the facts: for example, whether the crack only opens under strong flow, whether access/visibility was poor, and whether his agreed scope was only the visible leak he found first. That last part is judgment rather than a fixed rule. The legal standard is reasonable care and skill, not perfection.

So, on your three questions:

1) Should the plumber have spotted it?
Possibly, yes — if the remaining leak was discoverable during normal testing of the original repair. If he ran the taps fully and the U-bend still leaked, your position is stronger. If the second leak only showed intermittently or from a separate hidden defect, his position is stronger. This part is factual rather than black-and-white legal certainty. The underlying legal test remains reasonable care and skill.

2) Should you have to pay extra for him to come back?
Not if it is the original job not done properly. If the first repair was incomplete or inadequately tested, the normal remedy is repeat performance at no extra cost.

3) Can he charge because he says it is a “new leak”?
He can say that, but whether he is entitled to charge depends on whether it is genuinely a separate fault outside the original repair. If it is truly a separate defect that was not part of what he was engaged to fix, then an extra charge may be reasonable. If it is part of the same unresolved under-sink leak problem, you can challenge it and ask him to return it without charge.

What I would do next:

Write a short message stating that the under-sink leak was not resolved after the original visit.
Say you believe the issue should have been identified during testing.
Ask him to return and complete the repair at no additional charge.
Keep it factual: dates, what he replaced, when you re-tested, and what still leaked.
Take a short video showing the leak under high flow.

My opinion only and long post apology
You hired him to assess your plumbing, he identified the cause discussed this with you and repaired, when he completed that job im sure he would have checked to ensure the issue identified was resolved (im assuming you were happy he resolved the problem and paid him)
If it become apparent another leak has become visible now the (original) issue have been sorted, who’s to say this leak was visible at the time the plumber was there assessing the plumbing. This could have happened after the fact unless you phoned him when he’s 10 minutes down the road as most, do when the contractor leaves customers usually checks for themselves.

The work that he did originally has that failed if that hasn’t then I would deem this to be a new issue and yes a new call out.

Only my opinion so others may have a different take

I’ve had this with a loyal customer
2 years ago performed a repair all sorted.
4 days before Christmas just gone this leak appears, checked my work all good
Loose waste pipe due water coming from above tenant property. Reconnected job done all sorted charged

Following day phone call went round to check my work identified another issue causing a leak.
Problem sorted (charged)

Following day phone call it’s still leaking
Went round checked the previous work on both problems (no problems)
Identified another leak coming from the neighbours house who had some random washer waste pipe installed in the wall cavity of both houses that was causing a leak.
All three occasions my customer was charged for my time
Would you go to work for free to sort something out that you had already sorted.

Initially when you replce what he thought was the leak he should of tested to make sure yhay there was no other leaks and if there was he should of made you aware of it.

good morning

on first inspection i would have filled the bowl with water and then pull the plug so all the water drains with abit of pressure rather than just run a tap for a second or two,
secondly i would check the rubber seals in the flexi hoses (if fitted) or the fibre washers in the tap connectors.
final conclusion yes he should off spent time to investigate the issue and fixed it.if he says he has fixed it and charged then yes he should return on his own cost to solve.
hope this helps.
Regards

In situations like this, it depends on whether the original plumber properly tested the system before leaving and whether the new leak is genuinely unrelated or part of the same fault. He should always run taps under pressure to confirm there are no remaining leaks, so if the U-bend crack was present at the time, it should have been seen during the first visit. If that’s the case, then returning to complete the repair is generally considered part of the same job and should not be an extra charge. However, if the plumber can show that it was a separate issue or the crack was hidden and not visible during the initial repair, they may argue it is a new problem. Your best approach is to calmly explain that the leak never stopped, meaning the original repair was incomplete, and request a goodwill return visit. If they refuse, you may be better off using a different tradesperson.

It’s understandable to be frustrated in this situation. When a plumber is called to fix a leak, the expectation is that the system will be fully checked after repair — including testing under pressure — to confirm that no further leaks remain. If the original issue was misdiagnosed or not fully inspected, it would generally be reasonable for the plumber to revisit and put things right without extra charge.
However, if a new leak has appeared in a different section or as a result of an unrelated fault (for example, a cracked trap that wasn’t visible or caused later by movement), then a follow-up charge could be justified.
In cases like this, it’s best to discuss the situation calmly, ask the plumber to inspect and clarify whether the leak is from the same connection or a new point. A professional should always explain clearly what’s been found and why an additional fee might apply.
— Fixzer Plumbing

First of all, the plumber should have dried all of the sink and pipes and u bend trap. Then he should have inspected and then ran the sink taps to see where the leak was from. Just changing the seal is not good enough there should be a mastic or putty in between sink and chrome finish outlet that goes into u bend trap. The trap should been inspected too and taken off and cleaned while he was there as. Third of all but suspect for a new leak to happen so quickly… a good plumber or builder is looking for return custom and word of mouth(recommendations) he should of apologised and fixed the issues promptly.
If there’s a leak everything needs or should be inspected. You would not change the clutch in you car without looking at flywheel? While your there the mechanic should check fly wheel and make assessment while it’s open to change it……

The contractor that came to fix the leak should have checked all areas of the leak before saying completion of works was finished.

Additional fees should not be paid.

First question- the hired plumber should of tested the water to make sure there are no farther leaks.
Second question- If the leak is coming from the same spot then no you should not be forced to pay additional fee.
Third question- if proven that it was the same leak then try to convince him that it's his responsibility to fix the leak and if need more materials then that will be provided.