I found several of these worms amongst hydroids on laminaria. I only took photos of two worms. Worm 1 appeared to be budding. Worm 2 was approximately 0.5cms long.
No interactions present.
Lat/Lng: 51.7063, -5.0506
OS grid ref: SM893052
the worms were found on Laminaria which was attached to a jetty in Milford Haven, Pembrokeshire.