What puzzles me is why the joists have sagged I thought that ceiling
joists would normally be in tension - keeping the rafters from spreading and
maintaining the integrety of the walls.

I don't think they have sagged. The level is not very long and the house
is old. Rough sawn timber will shrivel a little between the knots. (I
bet the high spots are a metre apart -just the length to throw that
level out.

I think you could be spot on there. The removed ceiling definitely sagged.

Try a sting line underneath, right along them.)

OK