My image is curving down on the left side, I think it may be cause I'm off centre slightly and the projector is angled to the left to centralise the image. Does that sound right?
Any tips on getting it centre? I'm really governed by the joists though,
Yes that sounds right. You'll have to physically move the pj to the left a little and rotate it to the right and it should square up.
If that means moving the pj off of where there is a joist, can you get above the joists (from the floor above - lift a floorboard or two) and place a large flat piece of wood between them? A piece of MDF or similar that can lay the full width between the joists will be enough and give you a much larger area to screw through the ceiling plaster and into the wood. You can fix the wood using no more nails or similar (clean up dust etc from between the joists) or use a bit of wood each side of the MDF to hold it in place by screwing them to the joists to stop the wood moving as you screw into it from below (hope that makes sense).
I can't get in from above, osb sheet flooring which was miles out of level so I had to self level before I put down laminate flooring.
I might see if I can adjust the bracket a little more to see if I can pinch another 50mm.
I have art ex ceiling which I hate so I'm planning on over boarding with new plaster board. This way I can loose all my cables up there and also stop some sound going up, this may give me the opportunity to cut open and put in some supports to move it over.
It's hard to tell how much it needs to go over.
If you use a steel rule or a measured piece of string from the top corners of the screen which can be kept tight, you can use it to mark two arcs on the ceiling. The crossover point is where the center of the lens should go. Easier said that done, but might be reasonably accurate.