geoff was thinking very hard :
Can anyone give me a bleedingly obvious reason why two identical WCs with
maybe a metre difference in head are performing so differently?

The higher one seems to suck like a ... , and leaves only just enough water
to cover the "S" bend. The other one circulates and leaves a normal amount of
water in the pan

I don't actually want to get too involved (for obvious reasons0 but can
anyone give a few pointers as to why the first seems to have an over active
syphon?

The flow from the higher one will hit the bowl faster, making the speed
of exit faster. The faster exit once started will be slower to come to
a stop, so more of the water in the bend over flows before it stops.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk