I came across a canvas reproduction,with a frame marked Fiehl reproductions London.THe artist's name is Giovanni Canatella 1697-1768, with the painting's title being Piazzo San Marco.What sort of value would this reproduction be, if any?

Reproduction, do you mean a print of a painting? No value if it is a print, only if it is a numbered print and signed by the artist, which I doubt, they didn't number and sign prints 300 years ago.