Well, hopefully this will be my last post on this totally frustrating problem because I think I may have found the cause, which appears to be down to a faulty hard drive.
I ran the factory restore on my clone and it worked so I then reinstalled her original drive and did the factory restore, which also worked. As usual after a factory reset, there's now a lot of updates to do. Normally I'd use WSUS Offline Updater but for various reasons I didn't, letting Windows Update just download them this time and it reported that there were 177 updates to do (1GB-worth). It started installing the updates at 20.30 and still wasn't finished when I went to bed at midnight. When I got up in the morning it said that 157 of the updates had failed.
Back in with my clone on a known good drive and all updates completed successfully in a fairly short space of time, so I can only conclude that her hard drive is somehow faulty, despite all tests (GSmartControl; HDD Scan) all completing without error and showing a good drive.
