Thanks for the reply. I did as you instructed then reinstalled Tweaking.com. I then booted into Safe Mode, I selected the repair option, but left the Repair Network (and the UAC as I keep that switched off) unchecked.
I got the BSOD again concerning the rtwlanu.sys, but this time the message continued, collected some error info, then rebooted my PC back into Safe Mode.
I restarted the machine and I was soon back into Windows 10.
Later, I realised that there was something I was taking for granted.
I get my Internet via a broadband connection, but because of a problem I had with it a couple of years ago, I bought a wireless dongle, as a sort of backup. Could this be having an effect on the situation?
The dongle has been sitting in one of the USB ports on the computer. Out of sight, out of mind, so to speak.
As an experiment, I removed the dongle, then booted into Safe Mode and Tweaking.com.
I opted for the System Tweaks then rebooted the machine. No BSOD this time.
The following day, still without the wireless dongle, I Safe Booted into Tweaking.com.
I opted this time for a Disk Cleanup (got nearly a Gig from Memory Dump), then rebooted into Windows without a BSOD appearing.
I'll monitor this situation for a week or so, but at the moment I'm optimistic about the outcome.
Meanwhile, the wireless dongle will be kept in a safe place, 'just in case.'
Thank you for your help with my problem.
Gus, Edinburgh, Scotland