I have a solution!!!!!!
It works for the 'failed install/download' and the 'couldn't find obb data' issue.
After many hours of searching and trying different things I managed to piece together various bits of information and this is how I did it: -
1. Backup using Helium, there are various ways to back up but I used a PC and transferred back and forth from that. It also made it easier to browse the folders which is very handy for the further steps.
2. Delete Terraria from your device.
3. Install Terraria again and run it, just get it to the menu screen and then close it down.
3. Browse the folders on your device and find the Android folder, in there first go in to the obb folder and rename the Terraria folder, just add something like .bk to the end of the name, go back to the Android folder then in to the data folder and rename the Terraria folder in there.
4. Restore your backup in Helium to the new tablet.
5. Try and run it, within seconds it will error, close the app down.
6. Browse the folders again and go in to the folders as before, delete any new Terraria folders, rename the old ones back to what they were called (remove the .bk if that's all you added). If I re-call correctly I think it was only in the data folder that I had a new folder created.
7. Launch Terraria again and all your old stuff is there.
It is nice to know that I can repeat this process if neccessary because the 'failed download/install' issue has happened to my Son's versions of Terraria on 3 seperate occssions now.
I sincerely hope this works for everyone else too.