I did think that perhaps after a period of not being used and then transported that the mirror might be stuck, but it seems to move.
I say that because when I look through with only the prism in place I can get the image to go in and out of focus (changing size significantly at the same time) which is what I think I should expect?
The focuser seems to act along it full length of travel (or at least through its full number of turns)
I will try your idea, and then see if I can project an image onto some paper or something.
Right now all I get is what looks like a view of the inside of the eyepiece (rather like floaters in your eye if that makes sense) roughly illuminated by the source
Thanks