Hi Guys,
 
The good news is that I've just been testing my laptop for over 3 hours, recording sound from the TV and monitoring latency - it passed with flying colours!  I thought you might like to know that I found this link from Native Instruments that was very helpful and pretty much contained everything I could hope for on the subject:-
 
https://support.native-instruments.com/hc/en-us/articles/209571729-Windows-Tuning-Tips-for-Audio-Processing
 
I've edited the original heading to show that this is now solved and I've also edited the original post to show the solutions I implemented, without having to scroll down everything.
 
I do have a remaining side-effect that after I unplug my UA-25EX soundcard and go back to non-DAW work, I’ve lost the onboard soundcard (Realtek) driver, so I have no sound at all and have to restart, but I’m sure I’ll find a workaround or just live with it.
 
Thanks again for all your help!
Simon

Also, if the Realtek doesn't show up in device manager, it could be because you disabled High Definition Audio Controller (under System devices). I did that at first too, but then I discovered I didn't need to disable anything under System Devices to keep LatencyMon happy. Hope that helps.
Mark.

ryecatchermark
Also, if the Realtek doesn't show up in device manager, it could be because you disabled High Definition Audio Controller (under System devices). I did that at first too, but then I discovered I didn't need to disable anything under System Devices to keep LatencyMon happy. Hope that helps.
Mark.

Yes, I had High Definition Audio Controller disabled too and as soon as you do that, the Realtek Sound Device disappears from the list.  As soon as you re-enable the High Definition Audio Controller, the Realtek Sound Device reappears.
Solved.
Thanks Mark!