Lots of problems for you
I installed the GMIC package for Ubuntu 18.04 LTS Bionic : .deb package
If i run gmic in the terminal I get the gmic window popup, showing me version 2.57, so I can see this is installed ok…
Wrong package. That is the standalone command line gmic not the gmic_gimp_qt package, which is not going to work because of the QT version problem mentioned in previous post
…I then downloaded the GMIC plugin for gimp 2.10 and tried to manually place this in the correct path.
I have read and execute permissions of the file…
What makes you say that is the correct path, It might be, if you added it to the Gimp plug-ins paths.
You can find these in Edit -> Preferences -> Folders -> Plug-ins
However the place to put the gmic_gimp_qt plugin, is along with any other user plugin, in your Gimp profile.
For plugins this is in your home partition ~/.config/GIMP/2.10/plug-ins Remember ~./config is a hidden folder so un-hide in your file manager.
…gmic seems to be working, but the plugin is not, do you think compiling this is worth while? Or should I go down the route of updating QT?
Compiling is not difficult but needs lots-&-lots of additional development packages just for that 8 MB plugin. You might need to add some QT packages when you get the plugin in the correct place and find it does not register with Gimp. When working, the menu entry is bottom of the Filters menu.