From the horses mouth:
"Smoke (blue haze) noticed shortly after TOC from NCL by cabin crew. Captain informed and he personally had a look. Retuned to flight deck and started to consider options (get wx poss airports etc). Then told by cabin crew smoke was getting worse and pax upset.
Took decision to divert to STN. During descent smoke started to enter flight deck and O2 donned. After landing and considering a long taxi in with pax increasingly upset and smoke getting worse captain decided to evac".
It was assummed to be engine Oil burning and entering ducting.