Grum

Stephen - I think you are right. After doing a little hunting around, I found that only the male Sinister Moth has the feathery antennae - the female's are plain.
Many thanks for the ID.

During my SCUBA training, I asked my instructor if there was any chance of seeing these, and I was lucky enough that she took me to a dive site where we saw a handful of them in their natural environment. I felt very honoured to have seen these creatures that many people don't even realise exist.

Mona and SatyenM - thanks for commenting, and thanks to everyone for your faves.

Thanks Ali and Mona, and thanks all for the faves.

Thanks Sachin

Well done - finally got there after having everyone stumped for weeks :-[)

Thanks Argy (and Sarah too). Name amended.

Thanks Argy.

Thanks Guys. Unfortunately, just as the meal arrived, the adult turned away from me, and the juvenile went around the other side, so I missed that shot - would have made a good completion to the set... :(

I wonder if it might be some kind of Nudibranch or Sea Slug that has curled up on itself for protection while out of the water? Or maybe an anemone displaced from it's rock? How big was it?
You appear to have got EVERYONE stumped with this one!