I've posted once before about this issue but I've just had to change my cv boot again (the last one i put on a few months ago caught on the sump at some point and melted). Basically the driveshaft gearbox end won't fully locate, it goes in leaving a 2/3mm gap, last time I just put it in as best I could and there were no issues or leaks till about half a month ago. I gave it a good looking at and the thing stopping it going in any further is a solid metal bar inside the box which comes down at an angle just at the end of the internal spline. Any one have any ideas what that is or is that usually there? Should I be expecting a completely snug fit?

The solid bar you mention is the shaft for the planetary gears, it's normal for it to be there, try rotating the other driveshaft by hand, the bar should rotate when you turn the shaft... Try to centralise it with a clean finger then the shaft should pop in..

Don't be tempted to run it without the cir clip fitted to the inner joint spline or the shaft could potentially pull out of the box at full lock

Okay great, could i do this "reseating" without actually draining the bow and removing the shaft again, as I just put it back in as best I could? Maybe jack up the front end and rotate the opposite shaft periodically seeing if I can get the shaft I'm working on to go back in completely?

i doubt it will work but give it a try. it might save you having to fork out for another 2.5L of gear oil