Ok, after lots of testing/reinstalling I finally got to bottom of this issue. In summary:
1. Reinstalled backup image of Windows containing EIS 17.2.8
2. EIS updated to 18.0.11 & requested reboot. After reboot, EIS updated to 18.0.12 & requested further restart.
3. After reboot, EIS update would not run - always giving Module update error.
4. Activated VPN using server based in same country (U.K.) as my ISP. EIS update proceeded without error.
5. Deactivated VPN, switching back to my standard, direct connection to ISP. EIS update ran without errors!
So I think there was a glitch somewhere that prevented EIS 18.0.12 from updating via my ISP. Once the update had run via a VPN the error condition cleared, allowing further updates to be run on my usual direct connection. Go figure... !!!