I Have a new consumer unit with split load. The the whole system trips every time I turn on 1 particular up stairs light.

Andrews, January 2011

am sure that means a lot to you mate,
is it just one light?if it is,i doubt its a borrowed neutral,we norm wire light circuits on two rings,ground floor and top floor,and that means,if the neutral is wrong,all the lights would go out.
on that zone,check the bulb,

MM, February 2011

It's probably got a borrowed neutral, which should have been uncovered before the unit was changed.

Adam, January 2011