Hi Bernard,
A big fan of your books, I am currently reading Excalibur, which is just super. I actually think the Arthur series rivals Tolkien’s Lord of the Rings series and I can’t give you much higher praise than that?
My mail relates to the finding by divers of what archaeologists believe was a Viking Sunstone near Alderney. Apparently the calcite stone allowed  Vikings to navigate in foggy and inclement weather by virtue of the fact that when held up to the sky, the stone would reveal the position of the sun even before dawn or after sunset. I am wondering if you have heard of Sunstones and if, maybe you could incorporate the use of a Sunstone in the next Saxon story? You might find the following link interesting/useful.
Kind Regards
Willie
http://www.independent.co.uk/news/science/archaeology/not-just-the-stuff-of-legend-famed-viking-sunstone-did-exist-believe-scientists-8521522.html