I cannot thank all of you enough for this incredibly helpful (and given my ignorance, generously patient!) advice. I certainly won't purchase that sword, in any case. As I mentioned before, I'm a collector of Japanese antiques (mostly ceramics, fine arts, lacquerware, and fabrics) and more or less the same "rules" of connoisseurship apply in those areas, with the essential difference that good finds might still be made for exceptionally low prices, while it seems that one cannot purchase a decent nihonto for less than a (relatively) steep fee... As recommended, I'll also join the Token Society of Great Britain.
And by the way, my name is Pat - sorry for not specifying! No need for any titles