NavList:

Dave you wrote: "you could get a time when two stars were vertically above each other e.g. Canopus above Sirius, or Acrux above Gacrux."

Unfortunately I don't think that will work at the poles. From there the stars don't change their relative orientations but just describe circles of equal altitude around the horizon.

You also suggested: "You need a direction to set off to find the most recent of the black flags you’ve carefully left along your outbound track."

This reminds me an incident that occurred in August 1863 in the Central Otago, New Zealand stomping grounds of my youth. There is a monument at Gorge Creek in memory of around 35 gold miners who died having been trapped in heavy snows at the top of the Old Man Range. To prevent a recurrence the route to lower ground was marked with snow poles that were initially painted black. It was quickly realized that colours that don't reflect much light are not very visible and they were quickly painted red instead.

Regards,
Robin