[mole42uk]

After some more pondering, I think my mistake was to mis-interpret a data sheet. Pin 10 on that sheet is referenced as SHUTDOWN with a line above which I took to mean active low as in logic gate naming. However, it makes more sense in relation to the internal circuit of the 3524 if pin 10 is low to operate the chip, so SHUTDOWN with a line above means that if it is low the chip will operate, if high, it will shutdown. In other words, pin 10, in relation to the whole 3524, is active low.

From there, some of your analysis makes sense, line 1&2 are good, in that the chip is held in shutdown until the power input has stabilised, then the BC182 switches on and shunts the voltage to ground allowing R41 (5k6) to pull pin 10 low. The Service Manual states that this pin should be 0.3V.

Q10, the 2N6520, should be the over-voltage switch. The 75V zener D25 should allow the transistor to switch on at a voltage on R36 & R39 (both 39k) which is above 80V (from the Service Manual).

This supply is specified to run on any input AC from 100V up to 250V but my brain isn't yet able to work out how the power supply takes the 140-340V DC on the rail connected to PLD7,8 and make it into 70V DC to drive the main regulator since both are connected together on the drawing. EDIT, I've just looked at Bavarian Richard's drawing and realised that the power supply is messing with the ground to get the required voltages. How it does that is, as yet, as mystery to me, but if the main regulator ground is not at the same potential as the pre-regulator ground, then it begins to make sense.