Right then, before I could get round to finding white canes of the right length, they appear to have got better!
Don't know what we did to cure them, they spent a few days in the reduced pen, with plenty of grain scattered all over the floor, so that if they pecked, they had half a chance of hitting something edible, and they seemed to be able to find their way in of a night. Other than that, we did make a bit of a cocktail in their drinking water [little bit of antibiotic powder - yellow stuff left over from treating a viral chest infection type thing in the last chickens -, a bit of 'poultry spice' (to give them back their zig-a-zig-ah!), and some brewers yeast tablets, all mixed up]
Don't know whether it was the potion, or just time, but one, and then the other seemed to be getting a bit of vision back, so we opened one end of the pen to see what happened.
They seemed to be able to find their way out and back in of a night, and now appear to be back to their normal 'chicken' selves; pootling around, scratching, pecking and all the rest of it.
Our breeder friend had never heard anything like it either, and she's been hanging round chickens for ever pretty much. Her only suggestion was that we might have put something in the chicken area (soil, plants, articles painted with old paint or somesuch) that they pecked and temporarily poisoned themselves - though we do not know what it could have been.
Anyway, they appear back to normal, (though not laying yet) and so we'll just have to wait and see.
Since no one seems to have come across it before, I thought that I might coin a name for it, I was thinking of "Random temporary chicken blindness syndrome", but that seems a bit obvious, so I'll let you know when I have thought of something more interesting.
Thanks for all suggestions,
Nick