Michael
I think you are mistaken. From what I can see, the lead singer (Pete Wood) did not do the "plop". Look at it again carefully. Perhaps the plop came from someone in the audience.
I don't like people doing the plop - did this start because the Spinners did it? But I think some people do it ironically...
The thing about songs that supposedly get "done to death" is that they are good songs, and people like singing them. It does get slowed down too much in my experience, and my old mate Jim Mageean slows down verse 2, whereas the Wilsons singer (why can I never remember which is which...) speeds it up.
Derek