Unfortunately I cannot access my full library at the moment, however I do have a book showing a piece similar to the one in your photographs Steve.
I thought it was Victor Arwas's "Glass - Art Nouveau to Art Deco". I do happen to have a copy of the first edition of that book with me and it does not have this illustrated. It does, however, say under F.E. Rousseau in a passage on page 189:
"A number of other internal effects were achieved. These included various coloured streaks and patches and patterns simulating moss, lithen or autumnal branches. Colour patches were often used in conjunction with crackle glass."
I have also sold a piece like this as Rosseau many years ago. The foreign dealer who bought it from me confirmed my attribution at the time.
Whilst this is not conclusive, and I hesitate to come to the rescue of auction houses, I think on this occasion they may well have been right.
If the catalogue was Christies UK, as well as a Glass auction (as opposed to Decorative Arts), it is unlikely it would have sold - unless it was picked up by a continental collector or dealer - since it is not a range generally understood over here.
I would fully discount WMF as a contender. Incidently, although they did make a large number of Ikora "Unikat" pieces, there are pages and pages of Ikora production ware illustrated in the recent book on the subject.
Hope this helps, Nigel