Hi James
Are you sure your floor plate has been replaced? Reason I ask is that some HUPs, as I understand, had flat steel floors which were overlaid with plywood and linoleum. Which version HUP do you have? My 45 HUP has square dot pattern sheer metal in the rear area, and the more standard bar pattern diamond plate in the front area.
In the US you can get a vynal sheet footing that I've thought about using over the floors as sound deading.
Here is picture of my HUP rear flooring for reference.
Cheers Phil