A couple more sessions with R from down the road – he decided that weekends should be spent in his own garden (and fair enough too!), but he could drop past on his way home from work and render until dark. Tried it out to great effect one afternoon and got the other half of the front of the shed done – the only downside was that with me doing high stuff and R on the ground, my ladder always seemed to be bloody inconvenient.
Did a solo session the next time and did the high part ( > 2m) of the back wall. Next time R was going to come past I did a few loads earlier in the day and did the east wall high stuff, so when he rocked up we could both do ground-level without ladders getting in the way. Seemed to work pretty well, we got the whole eastern face and half the back wall done by dusk:
Still need to do the carport faces at some point, but at least the weather-exposed stuff is mostly done.