Hi,
Looking at that under the sink I would say to remove it all and start again with this kit below - McAlpine Double & 1½ Bowl Sink Kit 40mm White (71787) from screwfix.
I have never had any issues when fitting these for customers or any callbacks compared to when the customer has given me some none descript brand to fit.
Costs them more in the long run generally as I then have to come back and sort out the leak from the supplied trap by replacing it with one of these or similar McAlpine products.
Once this is fitted and joined into the existing plumbing you will have a good many years problem free operation,
Cheers,
Andy
P.S. Also need x1 of these - 85413 & 97562 as the one's fitted presently look like originally supplied parts with sink.