Can anyone explain why there's the blanket requirement that these are
fitted above the flood plain of the highest device?

I've got a short SVP (strictly I guess it's a sub-stack) with a Durgo on
it. Feeding the stack are a shower, bidet, WC and basin and I want to
set the top of the Durgo to be just below the basin to get a flat tiled
surface.
If the stack gets blocked there will be an unpleasant overflowing at the
loo, bidet and shower long before the basin gets affected so why do the
rules say the Durgo should be above the basin overflow?
--
Dave S
(The return email address is a dummy)