The largest of them apparently meausred around 25mm wide b 25mm long with the smallest around 10mm. They were characterised by a round mantle with a protrudng head and slighlty longer body below. They swam mostly upright using the mantle in a "fin-like pulsating manner".
After extensive enquiries they turned out to be a species known as Akera bullata which, although not too common, are normally found from Norway down to the Med. They're a gastropod, which is a mollusc, and belong to a subspecies known as Opistrobranchia.