OK this prompted me to discover more. These moon jellyfish (Aurelia Aurita) come into season in summer - so around now. However what 'in season' implies is the last stage in their lengthy reproduction cycle. From fertilised eggs stored in the mother jellyfish, the larvae - 'planulae' are released and free-swim for a few days before sinking. They create a colony attached to a solid surface. They become polyps and create new polyps also - and share a feeding system of tubes. These colonies can grow huge and sometimes remain at this stage for some years! In 'the season' with favourable conditions (which I imagine could mean warmth, sun etc) the top layer of this colony matures quickest and tiny individual ephyra (not fully formed jellyfish) peel away and float about while transforming into tiny jellyfish which gradually rise up to the surface - presumably what I am seeing). In a season only part of the hydroid colony will transform itself, the rest carries on at waiting stage.
Critically endangered leatherback turtles feed on jellyfish, so this would be the time for us to watch out for them as well as celebrate the wondrous life cycle of the Moon Jellyfish. Happy birthdaysssssss!