Yahoo!!!! Touch wood, I finally seem to have got the problem sorted.
I finally got around to watching the Youtube video below, which deals with diagnosing glow plug issues on the V6 engine.
Now, considering I had already replaced all the glow plugs and the control module, and checked the wiring loom, I was starting to feel at a complete loss as to what might be wrong, to the point where I was about to admit defeat and take it to a garage!
However, following the steps in the video I went through it all again and once more the glow plug that had been coming up on the diagnostic showed as the problem, despite having been replaced twice!
I checked an old plug, found it was alright, so swapped them over. Running the tests again, finally success!
So, if you are having glow plug trouble, I thoroughly recommend watching the video and following the steps shown.