Hi eso
Thanks for taking the time and thought to reply. I appreciate that 
As it so happens, the issue appears to have been fixed via an alternate approach. The problem was seemingly connected to a wider issue where Chrome (and related browsers) were also not downloading anything.
I raised this in a different thread, and rabcor and MyNameIsRichard offered suggestions which did work. These were to install xdg-desktop-portal and xdg-desktop-portal-gtk which had not been pulled in when I did the original Mate installation.
Solving this browser issue also (magically?) solved the Mendeley issue.
However, Iām starting a list of bookmarks and will add your suggestion to that just in case the solution is short lived or something else borks.
Thanks so much for your support.
Cheers