For anyone else experiencing issues. There are 2 main gotchas.
1. The path you have set up in tvheadend as the recording location, must be set up to automatically mount when you boot into Ubuntu.
It may well be that the drive will become accessible when you browse it with the gui file manager (nautilus), but this is because the system is mounting the drive on demand. This is NOT the same as automatic mounting at boot time.
If you don't set this up, you may run into issues if your box reboots and can't access the drive as a result.
You can find instructions how to do this here : https://www.binarytides.com/ubuntu-automatically-mount-partition-startup/
2. The second common issue is that tvheadend does not have the necessary privileges to access the mount and it's sub-directories.
This is the default for Ubuntu, so if you haven't set this up, tvheadend will fail to record, list completed recordings, timeshift, display logos etc.
My personal preference would be to have the entire drive dedicated to tvheadend or if you need other stuff on there, then it should have it's own dedicated partition. It's not mandatory, but it makes things more logical. Tvheadend needs folders for recording, timeshift and channel logos (or picons), so a dedicated tvheadend partition with a folder for each of these makes things nice and neat.
Whatever you decide, you need to give full access to tvheadend. There is a user account called hts (tvheadend's account). This is the account that needs to have access. You can do this via the command line as root, but if you are more comfortable using the file manager, you can do it graphically.
Type this in terminal to launch the file manager as root.
In the file manager, navigate to, then right click on the desired drives or folders and then select 'properties'. There is a tab where you can set permissions. You can set the owner and group as 'hts'.
I would do both. Give full read write and delete permissions to at minimum hts. Do this for the drive, all of the sub-folders and their 'contained files'. Changes made should take place when you close the dialog (there's no save button). Verify the changes have stuck by reopening the permissions dialog again, and check that they are as you set them.
To be 100% sure that you have given hts the needed access (don't just trust file manager), open a terminal, log in as hts and navigate into each directory. If you are unable to access the directories, you need to revisit above.
I am not aware that there is a password for the hts user, but you can work around this by logging in as root and using su (switch user) to get to the hts user prompt.
To do this, elevate to root by doing this in terminal:
Then switch user to hts:
You should now have a hts user prompt.
In this example, my mount is called '/media/Multimedia' and I have folders called 'RecordedTV', 'Timeshift' and 'TVLogos'.
Try all of the following from the hts user prompt:
'hts' should be able to browse into every folder in order for tvheadend to operated as expected. If not, change the necessary permissions.
Note: You can also change permissions in terminal as root using the chown and chmod commands instead of in file manager, if you are comfortable to do so.
Hope this helps someone! Any issues, just ask.