The DLR is not random ticket inspection. Every train has an operator who closes the doors, and checks tickets between stations. This was universal when it started with one-coach trains, when they expanded to two and now to three, which cannot be walked through, it is less so on shorter journeys, but the operator changes cars as you go along and if you travel end-to-end it is likely to be checked. I would say, looking at the checks, that 98%+ of passengers have valid tickets. This is probably as good as it gets, doubtless helped by the vast majority of travellers being "regulars".

Separately, there are occasional "snatch squads" who operate in groups, sometimes on trains and sometimes on exiting the platform. They are outsourced to a private security company who seem to have engaged a particularly thuggish lot (and whose command of even English can leave much to be desired - goodness knows what they make of the LCY European arrivals).

There is no love lost between the train operators, who have always been a reasonable lot, and the snatch squads.