Well sadly, this fish is no more. I just found him at the bottom of the tank
I'm not 100% sure it was a male as he was incredibly stressed and had been for about the last week so I'm not surprised that this has happened. I do suspect that he was a female as from the start he kept himself to himself, even in the bag travelling back from the lfs, he was at one end and all the others at the other end. I did try to get all males (which I may have still done) but I suspect this was in fact a female.
The others all look healthy though and are shoaling so maybe balance will finally be restored.