Thanks for all the help and suggestions. Since posting, we've taken the dining room ceiling down and done a lot of reading.
We've got two joists with rot issues. Further examination of the joists shows lots of knots in them. I counted 11 in one of them, most quite sizeable. There's varying numbers of notches in the joists, a good number in each and in the worst case 7. Notches of varying shapes and sizes.
The span is 2.660m and the worst deflection I've measured at 13mm with no live load (bath water, people) on top of the superimposed load (bathroom suite etc.).
According to current regs, the max deflection should be just under 8mm.
At the far end of the intermediate supporting wall, 2 of the joists are almost 10 mm higher than they are at the other end where seated in the exterior wall. In fact, it's quite a sharp rise and you can see the flex in the floor boards, one of which has started to split.
Temptation is to replace the joists with rot issues and replace the ceiling. But I'm seriously considering getting a structural engineer to take a look first.