Just a quick update for anyone interested or searching in the future;
Gearbox is on its way back from getting a rebuild, and from what I can remember this is what happened...
Low oil level, likely due to the "sealed for life" claim on the box and having never been done resulted in the pump having to word hard to keep the pressure correct. this has slowly flattened the valves. This has been why it was dropping out going into 4th and then going into limp mode.
The engineer doesn't understand how i managed to get it to shift in manual up through 4th 5th and 6th, he is guessing it was the very gentle approach to shifting that has allowed it.
Long story short, box required a almost full rebuild (opted to get a full rebuild anyway) and the torque was also knackered which is why I lost all drive.
Lesson to be learned, Don't mess about if the box is playing up, get it checked out ASAP. I'm looking at a total repair bill of around 3-3.5k (including shipping from and back to Shetland Isles)