PSU help understanding this circuit

Posting again as it appears not to have got through with a jpg attachment... I need some help with this . I understand how some of it works, but not all. Q1e is clamped by the 2v5 Vref.This provides a constant current for the 12V zener of ~ 2.5/780 = 3mA. The zener clamps Q2b @ 12v and thus Q2e @ 11.4 nom.This supplies some CMOS circuitry. So far, so good , I think. I know it looks like an overkill, but it's from a circuit that requires high reliability..... My questions are:- What is the function of the 3 x 51R + diode? Is it some form of current limit? And the purpose of the other 2 diodes? Temp compensation? I have one circuit where the Q2c is at 15V. What is failing ?Both transistors seem to measure passive diode drops ok... I'm guessing here, but I think if the load across the 11.4V is excessive, then that voltage drops in an attempt to limit the current into the load.... Any help here would be appreciated as my analogue understanding is really poor :( Circuit can be found here