Does anyone have any info. as to why the Marples, Fitzalan Square, was called the London Mart.

The Marples was actually called the London Mart at the time of the Sheffield Blitz. In the late 1880's it was known as Market Street Wine Vaults and owned by John Marples & Edward Marples. Despite a further change of name to the London Mart regulars always referred to it as Marple's.

This site will give you more info.
http://www.chrishobbs.com/index.htm

It was renamed "The Marples" after it was rebuilt after being destroyed in the war.

My auntie "Sadie Moore" sang there in the early 50s

Thanks, Chris Hobbs site looks interesting.