I see. The screws in the back are used to align the primary mirror. There is a process for that. I will see if I can find an article for collimating a reflecting telescope. It's been years since I had to do mine, I forgot the basics!
In your case if the mirror is affixed as an endcap then you can only cut the tube and remount the mirror...not likely something you want to do...all kinds of things can go wrong.
Other options are to replace or re-machine the focuser to be more low profile to get the camera closer to the tube. You can buy an eyepiece projection adapter which allows you to use an eyepiece in the middle to increase magnification or use a barlow lens which is kind of like a teleconverter for camera lenses but will also move the focus point out as well.
Reflecting telescopes can produce EXCELLENT image quality...often better than most camera lenses. However they are primarily only good for stationary objects and a pain to get focus exactly right. Good for the moon, landscapes, and paparazzi.
Thanks bkorcel. I just took a better look and on the main mirror there's some alignment screws and I'd hoped they might do the job but they don't have much play in them. Because it's more like an end-cap I wouldn't be able to just re-drill the screws without cutting down the tube. I'll pull it apart tomorrow and take a better look at how it works, I've got a small mill so might be able to make up something like some threaded spacers to hold the mirror further up the tube.
The odd thing is he said he used it with a 5D2 and had a sample shot of the moon so I thought I must have been doing something simple that was wrong. I know he could have been speaking crap but apart from seeming like a nice guy it was a local sale and I know where he works etc. Anyway in a few hours it'll be starting to get dark so might have a further play around and see how I go.