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no worries Duncan,
you will see a huge difference with more subs and using darks. then it comes down to processing which takes a lot of practice. Have you access to photoshop?
a dark is simply the same as a light sub - except with the lens cap on (so comes out dark). it records some of the noise that your camera creates. In DSS you select open darks and select them all. it will automatically subtract it (from the light subs) for you when processing. note that you need to use the same time and exposure eg 20 secs @iso 3200.
you are meant to do them around the same temperature so good to do on the same night. although a lot of people will use a 'master dark' for each season to save time (DSS will create a master dark [a stack of your darks in one picture] automatically if you have enough).
As many subs as you can get is the best rule, although I would say the magical minimum number of subs stacked would be 16.
don't be afraid to experiment a bit with times and iso on different targets.
you will have to click on my flickr page (see my signature) I have most of the details listed when you click on the pics.
A lot of people use the baader MPCC (multi purpose coma corrector) - it does a good job.
cheers
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