[rcheshire (Rowland)]

particularlarly if I open the ccd so different dust can get in, if it stays closed theoretically ( and in practical use) no change happens.

That's a good point. With DSLR's the sensor os frequently exposed to dust, so flats should probably be taken at every shoot.

There is an article here on the site about flats if you haven't read it.

http://www.iceinspace.com.au/63-211-0-0-1-0.html

Now I know what I'm seeing on my raws - the dark spots except for one have moved with use and are probably dust.