Thanks fellas, your help is appreciated.
The problem I was trying to solve is that the supply was working very erratically, if at all. Someone has attempted a prior repair, placing suspicion on some of the components and/or their orientation.
The unit is a dual-rail supply, output is normally adjustable between a nominal +/- 12-18v, but the trimmer was having no effect.
The 351 is configured as an inverting amplifier after the regulator to drive the negative pass transistor. The 723 drives the positive pass transistor directly.
A few components were not quite right. There was a 78L05 acting as a reference for the 351, supplying a fixed 5v to pin 7 (pin 4 being connected to the unregulated -27v rail). The 78L05 was inserted backwards and had failed. Not having the exact part, I substituted a 7805 the right way round, and things started to improve, although the voltages would still not adjust.
I also found that one of the electrolytic decoupling caps was reversed, so I renewed all three of those and fitted all of them in the correct orientation.
There are a couple of smaller transistors in the negative circuit that seem to be ok.
Voltage(s) are now fully adjustable, but the supply collapses under load and the base resistor on one of the output transistors is burning up. I have ordered new pass transistors and will replace those when they arrive. I have no way of knowing whether the Darlington output devices are correct, and since I will only require a couple of amps max from each rail, I have ordered TIP2955 and TIP3055 replacements to keep things as simple as possible.
If I can’t progress further I will let you know. I hate it when simple things get stubborn. ;D