Hi Simon. For astrophotography you would benifit heavily from getting a scope with an equatorial wedge mount, as opposed to an alt az mount like the cpc or nextar. The tracking mount is really more important that the optical tube when imaging.
It's the tracking. Tracking will be much less of a strain on the mount, and will be much more accurate with an eq mount like this:
http://www.celestron.com/c3/product.php ... ProdID=541
This setup shouldn't have any problems supporting the camera's weight.
Be sure that you polar align correctly. I don't know if you have any experience with astro imaging, but polar alignment can be a little tricky in the beginning depending on your exposure time. A .63 focal reducer/corrector could make things easier as well.