I am getting this line repeating in the journal every 5 seconds
Nov 27 10:45:35 separate_shot[3343754]: Failed to load module "appmenu-gtk-module": 'gtk_module_display_init': /usr/lib/x86_64-linux-gnu/gtk-3.0/modules/libxapp-gtk3-module.so: undefined symbol: gtk_module_display_ini
do I need to install/fix something ?
Maybe, @JimTR, but you can always start by trying:
sudo apt update && sudo apt upgrade
sudo apt reinstall appmenu-gtk3-module
Or
sudo apt purge appmenu-gtk3-module
(reboot)
sudo apt install appmenu-gtk3-module
Alternative solution: stop looking at the logs. 
This sounds like a package that hasn’t changed for quite a while, and an underlying library has - so there is now a mismatch.
A solution would be to recompile the package identified against the latest libraries in the ubuntu version you are using.
First is to identify which package is causing the issue.
Then to file a bug report on launchpad against that package.
Finally to a test rebuild of the package and install it to see if it resolves the issue.
I would but the journal had a size of around 8gb … which looked a bit big
I used journalctl --disk-usage to find out the info
is there an easy way to do that or do I just stop stuff until I don’t get the log entries anymore ?
The log mentions this - so basically need to-do an apt-rdepends to find any libxapp related package that has been installed. That will give a hint of possible candidate apps that need further investigation.
Clean up to keep only the last seven days of logs:
sudo journalctl --vacuum-time=7d
Have you tried what I have suggested?
In fact installing appmenu-gtk-module fixed the problem, oddly as this is a really old install there was a unity version installed that was giving the error perhaps there is a cut off somewhere on upgrades rather than doing a fresh install