Hi, and help!
I flashed the new ROM and it worked fine - see posting above yesterday.
Everthing worked fine until I rebooted the tablet.
Now it tells me 'Custom binary blocked by FRP lock' when it tries to boot.
I used ODIN to try to reload TWRP to start again, but that is also now blocked by FRP Lock.
How do I get back to a point where I can start again, and this time get TWRP to wipe everything.
How did that happen?
Anyway, you probably have to flash stock firmware via ODIN.. Then install TWRP.