I think this is a Compass Jellyfish, but not sure!A few coming in on tide this afternoon. This one was about 90mm across.
No interactions present.
A first for I-spot & a 1st 'sighting' for me!
See http://eol.org/pages/203481/overview ,or Wikipedia.
I wonder, Murdo, whether you should re-visit this and either assert your ID or agree with Chris - the latter will shift likelihood.
Lat/Lng: 58.4916, -6.2269
OS grid ref: NB537636