Thanks for the reply, gr5, some good tips there.
Winmerge says the files are identical, so transfer to card doesn't seem to be an issue.
I tried out Meshlab, and it looks like it could be a useful tool in the future, but when I tried applying it to my model, I got what looked like some strange artefacts and self intersection. I might just simplify my model to some extent instead.
I also went back to an STL that had worked previously, generated some new gcode from it, and that loads up fine. This does rather suggest to me that it's something to do with my model, and I have tried a few alterations to make it more printer friendly, but there's only so much you can do before compromising your aims for the design.
Does the printer take a long time to load large files? Am I being just being impatient (though I did wander off for a bit and come back, so I doubt this), or did your 76MB file allow you to select it immediately?
The model is a mix of thick and thin sections to the point where in a couple of places using the layer view, you can see slight overlap between red lines. I was under the impression that if Cura creates gcode, then the UM2 would attempt to follow it regardless of little issues like this, and they would just cause quality problems. I presume this is the case, and the UM2 doesn't check the gcode for anything shape related, just whether the gcode is readable and what flavour it is, correct?