Hi Henry,
Going over old ground here, but perhaps useful.
One little trick often forgotten that will help you considerably is to remove some of the multiple reflections. It is easy with the new SW collapsibe dobs - just place the fitted plastic dust cap over the mirror box.
With a solid tube Newt/dob -
1. Carefully sticky tape a sheet of A4 white paper just behind the secondary - effectively blocking off the main mirror - do this with the tube on an angle so the paper can't fall down the tube. Do not allow the sticky tape or paper to touch the mirrors of course.
2. Place a sheet of white paper directly opposite the focuser on the tube wall on the other side of the secondary.
The above will remove the many confusing reflections and blackness around the secondary holder that blends with the black tube.
3. Now you can put your Cheshire/Site tube in and easily see where the secondary is sitting and make your adjustments to the secondary holder along the tube, and/or rotating it as needed (NOT the 3 screws on the back of the secondary) to ensure the outer edge of the secondary is circular with the bottom edge of the Cheshire/Sight tube in the focuser.
4. Remove the paper carefully and now make slight adjustments to each of the 3 screws on back the secondary - Looking into Cheshire/Site tube make it so that you can equally see the 3 clips holding the main mirror in its cell in the reflection in the secondary mirror.
5. Now looking into the Cheshire/ Site tube adjust the screws at the back of the main mirror to bring the dot on your primary under the black dot created by the Cheshire/Site tube.
5. Don't put all your faith in the laser collimator unless you have set up a v block, rotated the laser and ensured the beam is exactly centred through the full rotation in the v block on a wall several meters away.
6. As Brendan correctly notes the Cheshire/Site tube is a very powerful tool.
7. Read the link below "Myths about Collimation" by the best in the business.
http://web.telia.com/~u41105032/myths/myths.htm
"If you center the secondary as seen in a sight tube, the offset toward the primary will be accomplished automatically, and you may not even be aware of the fact that it is optically offset."
And the all time classic "Perspectives on collimation" - someone in an astro club nearby will no doubt have this.
http://www.catseyecollimation.com/perspectives.html
PeterM.