Naturally I searched online for the solution, and many were suggested, but only the following one seemed to work for me:
"Unplug the tablet, uninstall the driver and restart Windows. Go to device manager and remove any remaining Wacom entries there, especially WacomVirtualHID under Human Interface Devices. Also remove remaining files wintab*.* from SYSTEM32 and SYSWOW64 (on 64bit Windows) as well as any wacom_tablet.dat or pen_tablet.dat files, which may still exist on the machine. Then attach the tablet to a USB port in the back of the machine, log in with an admin user account, then download and install the latest driver version."
As so often happens, some of these steps aren't so obvious. For example, "uninstall the driver". The driver doesn't seem to have a presence in Add and Remove Programs (there was something under "Bamboo", but Windows said it didn't exist and did I want to delete the entry. I did.) The "driver" seems to install into c:\Program Files\Tablet, so I deleted all these files. Before I did, I looked for an uninstall program, and didn't find one. However, there was a file called DPRemove.reg, which when double clicked removed many entries from the registry.
I don't know if it was important or not, but before the reboot (with the tablet disconnected), I did a search for 056A, and deleted registry entries that looked like they pertained to the tablet. Note that you get many false positives from GIDs; don't delete these or the consequences for the computer could be disastrous, and very difficult to repair. There were some registry entries that I could not delete; fortunately these didn't seem to be critical to getting the driver installed correctly.
In summary, the problem seems to be that various files from older Wacom devices persist in the system, and need to be cleaned out for the install program to work. This seems like poor programming on the part of Wacom, but it just doesn't seem to get fixed.
The new driver for me was this file: PenTablet_5.2.5-5a.exe (find the latest version via Customer Care on the main menu of the wacom.asia/au site, then Tablet Drivers. It is not accessible, as I had expected, from the product description page).
When all is well, with the tablet plugged in after the drivers are loaded, you should get the following in the device manager. Note in particular the circled drivers; without these, pen pressure in SAI doesn't seem to work.