Thanks everyone for your input. It looks like the general consensus is that the glow is pretty minimal and can be easily removed by subtracting appropriately-shot darks.
I'm going to go up in 5-minute increments now (still at ISO400) to 20, 25 and 30 minutes. Then I'll do the whole lot over again at ISO 800 and 1600. Phew!
I have used (to a varying degree) Registax and DeepSkyStacker, and to a far greater extent Photoshop CS3. Now that Houghy's explanations have prompted me to do a bit of research, I'll try the difference layer technique in Photoshop first. It is by far the most powerful image editing software available to mortals and should do a great job.
I'm having another deep sky session with the D40 over the weekend of the 9-10th November back at Magellan.
Cheers
Chris