Problem 'semi-solved'! To get it to work, the signal chain has to be as follows:
Variax to Helix Floor using the Variax VDI cable. (I use a Helix Stadium XL but I had to dig out my old Helix Floor as the Stadium has no Variax input)
Fully charged battery in the Variax with a TSR cable in the 1/4" output to a Vox AC15
USB from the Helix Floor to the desktop computer running Workbench HD
Both Central and Workbench recognised the Variax! (The legacy Monkey just doesn't function any longer hence having to use Central)
Workbench HD (according to Line 6) doesn't work with Variax guitars running anything older than Firmware 2.0 and so they need to be updated to the latest version. My Variax JTV 59 has been running Firmware 2.23 since I bought it and it is still not recognised by Workbench HD when trying to run a direct connection despite what Line 6 says.
This is what leads me to believe it is a software issue - something is stopping the Variax, the desktop and Workbench HD from talking to each other (they did last year!). My belief is it could be being caused by one of the many OS updates (Windows 11) and as Line 6 seem to be archiving support for Variax they have not released any updated versions of Workbench HD to maintain continued compatibility.
Thanks to everyone who has actively contributed to this thread and hopefully for some of the passive readers who might have had a similar issue they have also gained something from the commentary.