Yeah,definitely post a solution if you find it. I wish I could help more,but I really don't know what the problem is. I cant imagine its recovery related, but stranger things have happened. I certainly wish you luck!
Sent from my SM-G800F using Tapatalk
It WAS one of those stranger things! I updated the recovery to TWRP 3.1.1 flashed the ROM and boom - job done - Properly rooted. Screeen of Green
So conclusion - now we know. TWRP 3.0.0 flashed the ROM in a way which does NOT enable it to be rooted. wow.
Many Thanks for all the responses.