Why does it take so long to mend an escalator?

There are 409 escalators on the London Underground. At present about 95 per cent of them are operational at any one time. Statistics come with a whiff of smoke and a flash of mirrors. Numbers are a distraction when the grind of a necessary escalator is silenced. They do, however, tell you that one dead escalator in twenty is more of a nuisance than you would have guessed, and they put what is demanded of public-service escalators into focus.