I finally got this working...but what a "palaver"!
I sourced the TU needed but no matter what I tried it wouldn't work. So I deleted Infinity 3.0 (ripped from my retail disc via FSD3 into a folder). I then booted up my old RELEASE day 360, which suffers from intermittent RROD) , and after wrapping in a towel fro 10 minutes it was stable enough to run an update and then "install to HD" via the dash. I tried playing but it locked up a couple of times. It then gave the welcome msg that an update was required for Infinity 3.0. It installed ....
I took the drive off the box, connected to PC via FATXplorer64 and copied the Content and Cache folder. I then FTP'd these to the RGH box. Ran NXEtoGoD and it found Infinity 3 and converted it.
Looked in the dash and it had TU5 and TU6.
Works fine - so imho the issue was the original rip to HD not picking up the TU. My kids are happy now (TFA playset has been sitting unused since Xmas).