Is this the tiara you were thinking about Dave? Edit: Franck was the one to spot this tiara. Before that no one knew anything about it.
If so, then I don't think it is a match for the tiara worn by Princess Victoria of Wales to the Devonshire House Ball.
Princess Victoria of Wales at the Devonshire House Ball. Although the image below has a tear in it, it provides (I think) the clearest view of the details of the tiara.