Ok, so my nomadic nem has now, after 4 days of circling the tank, ended up back on the same rock and position as its bretheren.
I think I must have the smaller variety too. They all have the white stripes near the mouth, although the largest expands to about 15cms and doesnt have a lot of white fleck/mottling.
The 2 originals were small when I got them last September, one was about 3-5cms and the other about 1-2cm (they came on a piece of LR I got from a Masa member).
I think i may trade a couple soon........ but also may lose a limb in the process given the Queens (clown) penchant for them all!