I have now replaced the waste plughole fitting thinking the seal had gone and found that the water leak is from a pin hole in the actual sink so, any suggestions for a suitable sealant or patch to seal this underneath or replace the round kitchen sink??? I have thought of a patch to the underside maybe?? as it is just a pinhole

I cracked the plastic sink in my old folding camper & repaired it from underneath with a fibreglass repair kit from Halfords. It was a messy business, but the repaired section was stronger than the rest of the sink in the end.

thanks , should add that the sink is stainless

Not used it but this would be my starting point https://www.screwfix.com/p/unibond-36076...CFSQw0wodLQYBbQ

The above seems to tick the boxes looking at the reviews should do the trick.

Using it on the underside of the sink. Quick sand/file of the area to be applied to rough up a bit before applying a decent dab and spreading out a little.

No help at all, but how did you get a pinhole in stainless steel?

you are right, you are no help at all, not a bloody clue, must find a new location for pin the tail on the donkey