" to replace them our stone bench tops will have to be smashed "

Why do the tops need to be smashed? How are they installed? Pictures?

" Will they continue to deteriorate over time if left? "

Yes, the PDF is the finish, they will corrode most likely, what is the base material under the nickel PDF coating?

"Trouble is they are under mounted & to replace them our stone bench tops will have to be smashed which will involve damage to our wooden bench cabinetry."

The undermounted sinks are replaced with no damage or even removal of the countertops. The cabinet fronts are removed, the sinks replaced, and the fronts reinstalled. I've done this thousands of times.

I dont think there is enough room to remove them as we had hoped to replace them as per that procedure but we were told there was not enough room to move.

Picturees from underneath are what I meant as that tells us nothing unfortunately. It's entirely possible the person telling you has no idea what they are speaking on or the guy who installed the sink had no idea what he was doing and boxed you into a corner.

Maxine:

Please post a picture of your sink taken back far enough that I can see the countertop on each side of the sink and all of the doors/drawer(s). Like this:

Thank you for your assistance. Herewith some photos - from underneath (gap of about an inch between the sink & cabinet) plus photos of the sink set up.