Thanks for all the responses.
The problem with this job is frequency. We've done it several times on Carltons, Senators and Omegas. But the rear brake discs and the handbrake shoes last for years before they need replacing. So years pass before a repeat event and all the lessons learnt are forgotten.
We'd already suffered a delay staring the job and then got further delayed in getting the locking wheel bolts off.
The teeth on the adjuster cog are tight up against the spring, which also serves to prevent the cog moving due to vibration. When it's late and you can't see it properly it's all to easy to believe you're turning the cog when in reality it just snapping back to where you started from. Especially so when turning the adjuster wheel up and away from you, turning the adjuster down and towards you is easier and more positive.
We ended up pulling off the new disc, now easy to see the adjuster wheel and what it's doing. Then set the adjuster cog so that the drum of the disc just clears the shoes. Re-fit disc and back off adjuster cog. Much quicker.
A simple task of replacing the disc and pads became a saga. Incidentally the discs, pads and shoes are all GM parts and may well be original to the car. The discs are stamped 240203, a date-code I suggest.