Sloping trusses

I doubt I can answer your question but for the sake of clarity: when you say the ceiling in your attic room, do you mean that there is a (theoretically) horizontal section on each truss to which plasterboard is attached?
Is/are the ceiling/s in the room/s below also sloping?
Based on my very limited grasp of the facts, I would hazard a guess that whoever installed the roof perceived (rightly or wrongly) that the wallplates were out of horizontal and notched into them in an attempt to correct the discrepancy.
In terms of what you can do about it without needing a structural engineer, unless it's a newbuild under warranty or you think your surveyor has been delinquent, I suspect you will just have to lift the floor boards and make the lower joists up to the height of the others. Is it living quarters or just storage?

 

Possible the the block work wasn't level, leading to the gradual notching to compensate.

 

Would be unusual but maybe a discrepancy in the truss manufacture

 

How did you ascertain that it’s perfectly level?

It sounds like the brickies were a course of bricks low on one side of the house and the lads who put the roof on tried to reduce the discrepancy by notching the wall plate.

 

The brickwork is obviously out of level, the carpenters tried to compensate to make it less noticeable by notching the trusses into one of the wall plates, but not enough to sort it out.

The carpenters have not also not realised that if they put the truss clips on the outer face of the wall plate it would be easy to finish nailing them to the rafters!

 

Use your laser to take a datum through and around any doors or dividing walls, put a piece of masking tape on your walls if you don’t want to mark the wall with a pencil, then transfer the line from one side of the upstairs to the other.

 

If there is a 30mm height difference as per your original post and it’s not the floor joists, then it’s the ceiling joists. Walls parallel or not won’t make any difference to a level.