It seems that I have answered my own question about updating the list of Apps.
After uninstalling Calibre 5.18 (X86), you need to close Patch My PC and then restart the programme.
It the then finds and adds 'Calibre 5.18 (X64) to the list of Apps instead of Calibre 5.18 (X86)
After uninstalling Calibre 5.18 (X86), you need to close Patch My PC and then restart the programme.
It the then finds and adds 'Calibre 5.18 (X64) to the list of Apps instead of Calibre 5.18 (X86)