One found by searching hydroid samples at X5 to X16 magnification.
SEASEARCH N. Wales. June 8th-10th 2018.

Identification features:
Thompson and Brown (1976) regard the slender body, well developed pseudobranchs (item 7) and scattered dark crimson on the cerata (item 2) to distinguish it from other Doto spp.
1) Greyish body with sparse, dark-crimson, irregular spots; maximum length 14mm.
2) Cerata have elongate tubercles with internal, terminal, dark-crimson to black spot, plus sparse speckling of similar colour over their general surface. See above.
3) This specimen has very widely flared sheaths on its rhinophores, like the top of a milk jug; wide and rounded at anterior and pinched into a spout at posterior (but appearance varies with angle of view). See flic.kr/p/275YkMH (Thompson & Brown, 1984, state ‘sheath is slightly flared with scalloped rim’.)
The sheaths have crimson speckling, replaced by white pigment on the rim.
4) On this specimen, the front margin of the head is dilated into very prominent lateral flaps with rounded ends. See flic.kr/p/275YkMH . (Thompson & Brown, 1984, state “dilated frontal margin of the slender head forms a pair of small semicircular lateral flaps.”)
5) No red pigment marks on the inner face of each ceras. See above.
6) “Prominent longitudinal crests are visible in front of the rhinophore sheaths” (Thompson & Brown, 1984). In practice, these need viewing at different angles to discern and are not visible in many photographs. See flic.kr/p/27FYpPF
7) “Pseudobranchs are exceptionally well developed in D. millbayana” (Thompson & Brown, 1984). These can be difficult to discern in photographs. On this specimen they seem to be similar to a large tubercle, but with minimal colour, located on the mesial face of the ceras below the lowest circlet of tubercles. See flic.kr/p/28ZAbJ7