Andy, the 'top nut' (if you mean the one that the steering damper rod passes through) doesn't actually hold the forks into the frame. It is used for pulling down the 'top clip' (which is what Norton called the top pivot on the forks and more or less equates to what would be the 'top yoke' on a more recent bike).
This clip is actually secured to the fork stem both vertically and against twisting by a three piece clamping screw which has a wedge or cotter action. If you look underneath the bottom handlebar clamp half, it should be visible. Unfortunately, it seems that some owners don't understand how the system works and I've seen them replaced with a normal bolt which does absolutely nothing to prevent the forks dropping or twisting.
If you look at page 42 of the M&I (Maintenance & Instruction Manual) which is available for download on this webdsite (Top Resourece, Rob ! :-) )you can see a diagram of how it is assembled.
Making sure that the clamping assembly is tight before you release the top nut should stop everything dropping down, assuming that you have the correct parts.
To get the bars off, you need to remove the front two bolts which hold the tips of the U-clips, and also the four bolts (screws really since they fit into threaded holes) which hold the clamp over the rubbers. No need to interfere with the forks at all because the bars are behind the centre nut.
Putting the top clip back on is much more of a problem. The rubbers can be cut to go on round the bars, but they are rather fat and it's not easy to get the top bolts started. I used a G-cramp, but that's a big risk if you are proud of the paintwork.
