So, I think I’m happy with the results, even though I’ll need to re-print that base eventually, it was some old filament.
Parts list:
3D printed base, extender, and lock
Go Pro Thumb screws
Go Pro Tripod Screw adapter
Ball Head Tripod Stand
Tablet tripod mount
All in all, I spent about £25 (without the printed parts).
Just to complete the list, the tablet I’m using is a Samsung Galaxy S6 Lite, in a nice case.