does that mean you can't change the bog seat when it spoils? (without
removing the bog)

This is specific to the ideal standard washpoint loo and matching seat
- the seat works by having a pair of mounting pins that bolt through
the hole in the porcelain, and then the seat has a pair of thumbturn
clamps that lock onto the pins.

Disadvantage is you must remember to bolt these pins in place (and
position them correctly with the seat) before fitting the loo to the
wall, advantage is that the seat can very easily be removed for
cleaning.