Quick update!
Mantis is fine, he's now sharing the tank with a hermit (was 2, 1 is MIA), Some pulsing xenia (which grows very fast!), 3 turbos and all manner of hitchikers I couldnt name.
He's burrowed underneath most of the rocks so is doing ok in that respect. I wish I'd got sand looking back really, but there's not too much I can do about that now sadly, I may change it over in future if I decide to rework the tank.
I'm not running a skimmer so water changes are frequent, and there is a lot of algae. i'm not too bothered though, I like the look of a lot of it.
And yes, he's definitely a pseudosquilla ciliata.