Every time I look at a FPL for aircraft heading to LHR with MAN filed as alternate, I do have to scratch my head and wonder. I mean I know many carriers would have engineering/ground handling contracts in place, but how often are there no diversion notams for MAN?
Plus most airlines file the same alternates, so if AMS/FRA/CDG/LHR etc close, there's a fair chance you won't be going to your filed alternate/s anyway