A few more issues ...
Should be a 6V2 zener not 6V8 for best thermal drift, but in any case there is not enough current through it to work properly. Ditto the 30V zener. More current is a problem as its being pulled rather wastefully from 400V. 1V2 shunt regulators are good and cheap (band-gap) BUT that wouldn't leave any headroom for the long tailed pair. Is there a cheap / available ~ 10V shunt regulator ? Or even a 30V ?
Maybe Q2 would get killed by a shorted load
dc