Thanks for your suggestions Artie. Until my Mojave update, I've NEVER, in 25 years of Macery, had any problems whatsoever with a single aspect of macOS (Systems 6 - 10) which I couldn't solve once I'd removed the dumb and stupid hat.
Thanks also for the 10.14.2 Combo link; I've downloaded it and it's awaiting action. I've read many times that a Combo will often solve problems but have never needed to run one. I presume I can do so over the top of the existing Ap Store sourced Mojave. (??)
I used "sudo rm -R" on the recalcitrant file but this did nuffing. I have since read of several others like rm -rf ~/.Trash/*, sudo rm -rf ~/.Trash/*, sudo rm -rf /Volumes/*/.Trashes, and sudo rm -rf ~/.Trash /Volumes/*/.Trashes. However I think these are well and truly above my confidence/aptitude level and may make it worse. Because...
...I ran Disk Utility > First Aid from the boot volume which failed and told me it had found corruption (such an emotive word) that needed to be repaired and advised repair from First Aid in Recovery mode. This failed too. Far out. It's possible that this corruption was caused by the MS techos when they took over my machine remotely to solve the Office issue. It was certainly entertaining watching them flail around!
I think it's time to run the Combo.
