It's extraordinary that the original P86 cars are still running in Essen, while the trains ordered to replace them are themselves now up for replacement. Using them constantly I actually don't find anything wrong with them. It seems to be once again TfL profligacy. The trains being replaced have a better seating arrangement than the ones that came for the Olympics (always a disappointment when I see one of these coming), and the new ones now seem notably worse again.
I'm reminded of the US President's helicopter, where a replacement project started off and rapidly got to a cost of many billions, and I think they were going to have a fleet of more than 20. President Obama finally stopped that one with the comment "I don't see what's wrong with the current helicopter, it seems fine". And it was. And 10 years on, it still is.
Probably late 1990s, when the Beckton line first opened as a shuttle just to Poplar.