Gparted not working: "Invalid partition table on /dev/sda --wrong signature 0"

[m654321]

Solved it 

First, I can get into gparted by pressing the ignore button on the Libparted Error notification - silly me!
Within gparted, I then discovered the source of the Libparted Error: when I cloned the dual-boot system from the smaller to the larger drive, the extended partition remained at its former size.  So, I therefore extended this to the rest of the drive, which includes the new unallocated space on the larger drive.

Gparted now opens as normal, without any signature error or any other issues - problem solved - oh happy days ...