Post scriptum 22 November 2011: I visited last week the Botanical Garden of Barcellona, and was happy to be able to confirm my species diagnosis. It's Scilla latfolia. It's a nice species growing on Tenerife and Lanzerote. So it's clear for me that I have an additional potted plant, this species will not survive our winter temperatures:
Scilla latifolia at Barcelona Botanical Garden, note the previous inflorescences with aboundant seed capsules.