I would say it's definitely cupanii... I don't think there are any other options than cupanii and psaridis, which are easily distinghuished in cultivation by the stoloniferous underground bits of psaridis. I don't know of any other species in the cupanii group that have wide leaves that are so well-developed at flowering time. Identifying Autumn-flowering colchicums that have leaves at flowering time is relatively easy compared to identifying those that develop leaves later in the season, the choice is very limited in the first group.
There is a very good illustration of psaridis, including the underground bits, in the journal of Scottish Rock: Kit Tan & G. Iatrou, The Rock Garden, June 1995, “A new Colchicum from the southern Peloponnese”.
Anthony, the leaf width in cupanii can be quite variable, some forms have much narrower leaves. There should be no more than 2-3 leaves though.
By the way, do you think I could try cupanii outside here in the Netherlands, or is it susceptible to frost?