SD - thanks for your reply;
Task scheduler isn't ideal as shutdown happens variably in a 1 hour or so window & the computer isn't left on site (so, scheduled task cant be set up to occur later), so ideally it would be driven by shutdown & not a specific schedule.
Acronis can be set up to shut down automatically at backup completion, but this would require actively starting Acronis before shutdown & in this scenario it unlikely to happen...