Thanks for your suggestions. I have now managed to get it going.
I'm not entirely sure where the issue lay but, one possible key fact I forgot to mention was that in order to uninstall my previous version 5.6 I had to resort to the Microsoft FixIt: "MicrosoftFixit.ProgramInstallUninstall" utility to remove it as the normal uninstall procedure kept giving errors saying the msi installer file was not valid.
What I hadn't done though was also uninstall the 5.6 Viewer at the same time. I did that this morning and once again had to use the FixIt uninstaller. Once that was done, I removed all traces of the Remote Utilities folders from the Program Files and Application Data areas(there were no registry keys there as I'd removed them on a previous attempt).
So the key difference was either eradicating all of the (possibly corrupt) Viewer prog too, or deleting those program files from their folder (or both).
Thanks for your efforts.
Hopefully this will help someone else.