Hmmm.... this is embarrassing!
I was SO sure that it was a valid alternative spelling that I didn't do a comprehensive check. I did do a quick Google and got quite a few hits, including the page title text on the "British Litchen Society" - but on rechecking I find that even that page uses "lichen" in the body of the site.
And a couple of online dictionaries don't have it - so I checked my bookshelf and nor do my reference dictionaries.
So, folks, it appears to be a common misspelling, and one I was guilty of.
I shall retire quietly to hide my blushes and beg you all to forgive me and my hasty post this morning.
Sorry.