Confidence: I'm as sure as I can be.
Notes: I've photod it 3&a bit times,(in the Gower) & they were virtually identical.
SIZE I still have the same size & make container so in Pgdwm22 the"0.2L"=.6cm so making the worm .9cm.
The (Collins) Sea Shore 1996(Pocket Guide) colour illustrnP.89 is pretty identical (shape,tentacles,colours), but gives size=10-15mm & says 15 segments where my two have about 12 & no end segment. But the text also notes that the parent Syllidae sometimes reproduce by developing into a surface swimming stage, the Epitoke. This chimes with my 2 which were found swimming at the surface & also have a 'constricted/normal' front end & a 'bloated' back end.
Examples of Amblyosyllis or A.s. in Google pics show the form but not the same pattern &of uk marine sites only species-identification.org has it (a drawing). The WoRMS (World Register of Marine Species) site has it,& mentions there are only 2 sp's of Amblyosyllis recognized, but of the other, A.madeirensis (no illustration anywhere),it's regions include the UKEEZ but oddly only A.f. is in the British Isles Polychaeta list.
I've christened it "pagoda worm" (just quoted 1ce in google, Wm Beebe 1928 )