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It shouldn’t be a difficult fix for you to find the leak if you’re comfortable taking it apart, but if it’s a poor install then as you say get it sorted for free.
You’ll need a big enough adjustable to get it apart and a joint sealant and silicone sealant to get it all back together.
I’d check the right type of waste was used for your sink - there’s slotted and un-
slotted, the former will leak for fun if it doesn’t match your overflow. And check that the right size gasket was used.There’s nothing complex there. Both joins you point too need to be slathered in Plumbers Mait or preferably LSX to make them watertight.
I have a leak, in two places underneath a newly installed sink.
Obviously I'll get the plumber back in, but in the meantime, should I tighten or loosen any of these parts?
Leaks are at the two red arrows.